GENERAL TOPICS
- Study of following with reference to:
2.1 Arthropoda: - Mouthparts in Insects, Metamorphosis in Insects, Mimicry in Insects, Economic importance of Insects, Larval forms in Crustacea
2.2 Mollusca: - Economic importance of mollusc, Shell and foot modification in mollusc, Torsion and Detorsion in mollusc, Larval forms in molluscs
2.3 Echinodermata: - Origin of Echinodermata, Types of Pedicellariae, Larval forms in Echinodermata,
2.1 ARTHROPODA :
2.1.1Mouthparts in Insects
Insects feed on animals and plants in a diversity of ways and their mouthparts have become modified for these purposes. The mouthparts are essentially the paired appendages of the mandibles and the maxillary and labial segments of the head. These have evolved into a variety of forms, which have been perfected to meet different kinds of highly specialized feeding habits. The important types of insect mouthparts can be described as follows:
I) Biting and chewing (Mandibulate) mouthparts
The biting-chewing mouthparts represent the most primitive and unspecialized type from which all other types of mouthparts have evolved. They are meant for pinching off, chewing up and swallowing the piece of plant and animal tissues. These consist of the labrum or the upper lip, a pair of mandibles, the paired maxillae (first maxillae), the labium (second maxillae) or lower lip, and the hypopharynx. These are described as follows:
1) Labrum: It is also called as upper lip. It is simple and variable in shape and size, lying in the front of the head hanging from the clypeus. Its function is to open and close the mouth.
2) Mandibles: A pair of mandibles lies in the cavity of the mouth. Each mandible is an unjointed triangular piece, attached dorsally and bearing teeth-like projections on its inner margin, called as denticles. They work transversely to masticate or grind the food. The mandibles, worked by two sets of muscles, masticate the food with their tooth-like processes.
3) Maxillae: These are attached one on either side, laterally. Each maxilla consists of a basal part, made up of two segments, a proximal cardo and distal stipes. From both the sides it carries two long, tubular structures called as maxillary palps. The maxillary palps act as sensory to locate the food. The lacinia is often employed for grasping the food and cutting or chewing it. The maxillae work transversely for catching or holding the food.
4) Hypopharynx: It is a median process, also called as tongue. It is lying in the buccal cavity arising from its floor. The opening of the salivary duct lies at its base.
5) Labium (second maxillae): It is also called as lower lip. The dorsal part is made of two segments, the mentum and the sub-mentum. The mentum bears distally two labial palps, one on either lateral side. Each palp is usually 3 jointed. In between the palps there is ligula formed by the paired glossae and paraglossae. These help in pushing the food into the pharynx.The labial palps are sensory and help in testing the quality of food as they contain chemo-receptors.
Examples – The biting and chewing types of mouthparts are common among Orthopteran insects such as grasshoppers, cockroaches, beetles, termites and crickets. They also occur in silverfish (Lepisma), earwigs, termites, book-lice, bird-lice, beetles, some Hymenoptera and many larval forms, specially the caterpillars of Lepidoptera.
II) Chewing and lapping mouthparts
This type of mouthparts is found in some of the Hymenoptera (bees and wasps) and is modified for collecting nectar and pollen of flowers and moulding the wax. These mouthparts serve for both biting and licking.
The labrum lies beneath the clypeus and the fleshy epipharynx projects below the labrum. The mandibles are situated on either side of the labrum and are smooth and spatulate and the workers use them in building the honeycomb. In the maxillae, the lacinia is absent. The maxillary palp is vestigial but the galea has become elongated and blade like. The labium is jointed basally to the triangular postmentum or submentum. The paraglossae are greatly reduced while the glossae are united and hairy and greatly elongated forming the retractile ligular tongue, which terminates distally in a honey spoon or labellum.
While feeding, the galeae of the maxillae and the labial palps are brought close together forming a temporary hollow tube enclosing a food channel. This tube can be inserted deeply into the corolla of a flower. The glossae move backward and forward to collect the pollen and sucking up the juices of the nectars, assisted by the pumping action of the pharynx.
Examples – Honeybee, Bees, Wasps
III) Piercing and sucking mouthparts
- The insects possessing such mouthparts are fortunate enough to have the most valuable liquids in the world, because they almost live on the sap of the plants or the blood of many mammals, which they suck after making a puncture in the epidermis or skin of the host.
- Various modifications of these mouthparts are seen in bugs, aphids, mosquitoes, flies and fleas. The labium forms an elongated, fleshy and mid-dorsally grooved hollow protective tube or proboscis. Its distal tips bear two small labellae, which are used as feelers and enable the mosquito to select the appropriate part of its victim to attack. The labrum is long, needle-like, fused with the epipharynx and covers the mid-dorsal opening of the labium. The mandibles, the maxillae and the hypopharynx are also elongated and pointed distally forming needle-shaped stylets for piercing the skin of the host. The hypopharynx is double edged, served with a mid-dorsal groove, which forms the salivary passage. While feeding, the hypopharynx and labrum become closely applied to enclose a tubular passage for the ascent of blood.
- Examples – The piercing-sucking mouthparts are found in blood sucking insects like the mosquitoes, fleas, the bugs such as bedbug and kissing bug, and the herbivorous insects such as aphids, which feed on plant juices.
- In mosquito, the mouthparts consist of a long proboscis or beak, which is composed of the labium, forming an elongate, fleshy and mid-dorsally grooved tube. It encloses the needle-like stylets formed by the modifications of the mandibles, maxillae and hypopharynx. The needle-like labrum is fused with the epipharynx and forms the long covering of the open groove of proboscis. The proboscis bears at its tip, two small labellae, which used as feelers and enable the mosquito to select the appropriate part of its victim to attack. These mouthparts are well developed in female mosquitoes as they feed on blood.
- In bedbug the labium forms a three jointed proboscis. The stylets are four in number, consisting of two mandibles and two maxillae, the former with blade-like and the later with saw-like tips. The labrum is a flap-like structure, covering the groove of proboscis at the base only. Of the four stylets the maxillae are doubly grooved on their inner faces, one acting as a food canal for the flow of blood and the other as a salivary canal for the flow of saliva.
IV) Sponging mouthparts
Sponging type of mouthparts are found in the common housefly and others, and modified for sucking up the liquid food. The mandibles are altogether absent, while the maxillae are represented only by two maxillary palps, each made of a single piece. The labium is greatly modified to form the proboscis, which is divisible into three parts – (i) a proximal cone like rostrum bearing the maxillary palps, (ii) the middle haustellum with a mid-dorsal groove serving as the food passage and (iii) the distal labellum consisting of two expanded lobes or labellae.
Ordinarily, the two labellae lie retracted upwards and backwards, but when the fly feeds they are protracted, expanded and placed over the liquid food. If the food is a solid material of an easily soluble kind, such as sugar, the fly exudes a drop of saliva to dissolve it. The liquid food is first collected by the pseudotracheae and then passed onto the food canal which is formed by labrum and hypopharynx, lying in the mid-dorsal groove of the haustellum.
Examples – Housefly, Blowfly, Fruit fly
V) Siphoning mouthparts
The siphoning mouthparts of butterflies and moths are highly specialized for sucking up the juices of fruits and flowers.
The mandibles and the labium are very much reduced. The maxillary palps are vestigial. The labium is represented by a triangular plate bearing the labial palps. The essential working parts are formed by greatly elongated galeae of the maxillae, each in the form of a half tube. When applied together, the two galeae form a complete, slender, hollow tube called as proboscis. When not in use it remains coiled up like a watch spring and held close o the lower side of the head often concealed by the abundance of scales. When the insect wants to feed, the spiral proboscis is uncoiled and extended to reach nectars of flowers. It is the rise in blood which uncoils the proboscis.
Examples – Butterfly, Moth
2.1.2 Metamorphosis in Insects
Insects usually hatch in a condition morphologically different from that of the adult. Consequently, they have to pass through changes of form which are collectively termed as metamorphosis. The term “metamorphosis” is derived from two Greek words (meta – change; morph – form), designating a change in form.
Insect metamorphosis has been defined as the transformation of an immature larval individual into a sexually mature adult of different form, structure and habit of life. According to another definition, metamorphosis refers to abrupt changes in form from one distinctive stage to another in the life history.
Insects display 4 types of metamorphosis –
1) No-metamorphosis or ametabolous development – In case no- metamorphosis, newly hatched creature looks like an adult except in size and differences in armature of spines and setae.
Examples – Silver fish, Spring-tails
2) Incomplete metamorphosis or hemi-metabolous development – In case of incomplete metamorphosis, immature stages are the nymphs, which are aquatic and respire by tracheal gills, whereas the adults are terrestrial or aerial and respire by tracheae.
Examples – Mayflies, Dragonflies, Stone-flies
3) Gradual metamorphosis or pauro-metabolous development – In case of gradual metamorphosis, the newly hatched creature resembles an adult in general body form, but lacks wings and external genital appendages.
Young or the nymph undergoes several nymphal stages through successive moulting to become an adult.
Examples – Grasshoppers, Aphids, Stink bug
4) Complete metamorphosis or holometabolous development – This type of metamorphosis includes four developmental stages – egg, larva, pupa and adult. Larva, after hatching, moults several times to become a fully grown one. It later becomes a pupa within a secreted case, called the puparium. Pupa differentiates into the young adult that breaks the puparium open and emerges outside. It grows to a mature form.
Examples – Housefly, Mosquito, Butterfly
2.1.3 Mimicry in Insects
Most insects are quite vulnerable to predation. If you can’t overpower your enemy, you can try to outsmart him, and that’s just what mimicry in insects do to stay alive. As humans, when we see signs in bright yellow or red, our minds instinctively tell us to pay attention, stop or think with caution. Insects and other animals with bright colours of certain patterns use this same idea to their advantage to warn those that might harm them.
Mimicry is the similarity of one species to another to the benefit of one or both species. Two types of mimicry include Batesian and Mullerian, although these forms of mimicry should be considered two ends of a spectrum with various forms of mimicry in between. In Batesian mimicry, the insect is essentially a sheep in a wolf’s clothing in that the mimic (a palatable species) parasitically resembles a model (an unpalatable species) (Balogh et al. 2008). Mullerian mimics, on the other hand, involve two or more poisonous species that have evolved to mimic each other’s aposematic coloration or warning signals (Balogh et al. 2008).
i) Batesian mimicry
In 1861, English naturalist Henry Walter Bates first offered a theory that insects use mimicry to fool predators. He noticed that some edible insects shared the same coloration as other unpalatable species. Because predators learned to avoid insects with these colour pattern, he argued, the mimics gained protection by displaying the same warning colours. This form of mimicry came to be called as Batesian mimicry.
In Batesian mimicry in insects, an edible insect looks similar to an aposematic, inedible insect. The inedible insect is called the model, and lookalike species is called the mimic. Hungry predators that have tried to eat the unpalatable model species learn to associate its colours and markings with an unpleasant dining experience. The predator will generally avoid wasting time and energy catching such a noxious meal again. Because the mimic resembles the model, it benefits from the predator’s bad experience.
Numerous examples of Batesian mimicry in insects are known. Many insects mimic bees, including certain flies, beetles, and even moths. Few predators will take the chance of getting stung by a bee, and most will avoid eating anything that looks like a bee.
Birds avoid the unpalatable monarch butterfly, which accumulates toxic steroids called cardenolides in its body from feeding on milkweed plants as a caterpillar. The viceroy butterfly bears similar colors as the monarch, so birds steer clear of viceroys, too. While monarchs and viceroys have long been used as a classic example of Batesian mimicry, some entomologists now argue this is really a case of Müllerian mimicry.
ii) Mullerian mimicry
A German naturalist Fritz Muller offered a different example of insects using mimicry. He observed communities of similar colour insects, all unpalatable to predators, and theorized that these insects all gained protection by displaying the same warning colours.
Mullerian mimicry occurs in nature when two or more harmful species look very similar in order to ward off potential predators. This is very advantageous to animals as a means of protection. If animals that resemble one another are all known to be poisonous or dangerous, they will have a significant advantage because predators will quickly learn to avoid them. One common example of Mullerian mimicry can be seen in species of butterflies. Heliconius erato and Heliconius melpomene are two different species of butterflies that exhibit Mullerian mimicry. Both of them have evolved to have mostly black bodies and wings, but they have a similar pattern of red-orange dots and markings on their wings.
2.1.4 Economic Importance of Insects
Love them or hate them, we need insects for global survival!
Most people think that insects are pestiferous creatures and should be destroyed at all cost. But few people know the exact nature of damage they do. Even fewer people know the many acts of insects that are indispensible to man. Unfortunately, beneficial acts performed by insects are far outweighed by the tremendous amount of damage done by them.
I) Harmful or Injurious insects
1) Pests of plants, fruits and stored grains
Insects attack leaves, stems, buds, flowers, seeds, fruits, barks, woods, roots, vegetables, stored products, wool, feathers, cigars, tools and even minerals. Chewing insects, like the cabbage-worm, hoppers and potato beetles, chew and swallow the external parts of the plants. Grasshoppers and locusts have invaded the green crops since times immemorial. Locusts, on the war-path, sometimes move on in swarms extending for many kilometres. Grass and leaves are devoured and even branches are broken by weight of insects settling on them. Plant-bugs, aphids, scale insects, possess an extremely sharp pointed, beak like process or proboscis, which is inserted into the plant tissues to suck up their juices. Bark beetles destroy timber in the forests, while termites’ damage timber after it leaves the forests. Boll weevils spoil cotton before the harvest. Caterpillars and Japanese beetles strip the foliage from millions of shady trees every year.
2) Household pests
Several insects are unwanted guests in the house. Mostly annoying, sometimes they become destructive. Bedbugs, mosquitoes and stable-flies are much annoying. Ants, crickets, cockroaches, weevils, fruit-flies and silver fish etc., spoil the food. Clothing, carpets, furs and feathers may be damaged by cloth moths and carpet beetles etc.
3) Injurious to domestic animals
Domestic animals are often seriously injured by insects. Many of them live more or less as parasites either externally, such as fleas, lice, bugs, mosquitoes etc., or internally, such as larvae of botfly in sheep. The bird lice (Mallophaga), feeding upon feathers of chicken, cause irritation and loss of flesh. The blood sucking horn fly is a serious pest of cattle. The grubs of ox warble fly cut holes in the skin of cattle, thus causing damage of hide and flesh. The larvae of horse botfly sometimes cause serious disturbances in stomach.
4) Disease carriers
Many insects play a great role in spreading serious diseases of man and domestic animals. They often act as vectors for transmitting various disease producing organisms either by infecting them in blood stream e.g. malaria, or by contaminating the food e.g. bacteria. The various species of the genus Anopheles of mosquitoes have been found to convey one or other parasitic Protozoa causing human malaria. Certain Culicine mosquitoes spread the nematode worm, Wuchereria bancrofti, which causes Filariasis in man. Similarly, yellow fever is spread by a mosquito Stegomyia; Surra disease among horses, camels etc. of tropical countries by Tabanus fly; African sleeping sickness by Tse tse fly; Typhoid, diarrhoea, cholera, etc. by common housefly Musca domestica; Bubonic plague by fleas and relapsing fever by bedbug and body louse, and so on. Undoubtedly, the insects carrying diseases are greatest enemies of man, affecting human welfare most profoundly.
5) Poisonous insects
Many insects and larvae produce poisonous secretions, which are injected into the body of man and other animals either through a bite or sting. Irritation of skin pain and swelling may result. The common examples of poisonous insects are honeybees, wasps, hornets, fire ants, bedbugs, mosquitoes and a few lepidopterous and other larvae.
II) Productive insects
Human beings are greatly indebted to certain insects, which supply them with useful products. Many commercial products produced by insects are indispensable to modern man.
1) Honey: Honey is produced by honeybees (Apis). In U.S.A., 6 million colonies of honeybees produce about 150,000 million tons of honey annually which serves as human food and medicine.
Honey is one of the beehive's principle food resources. It is produced from droplets of flower nectar gathered by worker bees. The nectar is temporarily held in the bee's foregut where enzymatic action begins to convert sucrose into dextrose (glucose) and levulose (fructose). In the hive, this nectar-enzyme mixture is transferred to waxen cells, reduced in volume by evaporation of water, and allowed to ripen into honey. The bees seal each cell with a wax cap when the process is complete. Worker bees make as many as 50,000 trips to and from the hive and visit up to 4 million blossoms in order to produce a single kilogram of honey (2.2 lbs). Large, healthy hives may average more than 25 kg (55 lbs) of honey per year. Although the market for honey is not as large or as profitable as it once was, annual U.S. production is still over 115 million kg. Most of this honey is used as a primary sweetener or as a substitute for refined sugar in baked goods. It is also an ingredient in a few cough medicines and laxatives.
2) Bee wax: Specialized glands on the ventral side of a worker bee's abdomen secrete flakes of beeswax, a soft, malleable material that bees use to build the comb where honey is stored and larvae are reared. The wax has a relatively low melting point so it is easy to extract and purify with heat. Beeswax is still used commercially in the manufacture of cosmetics, candles, furniture waxes, leather dressings, waxed paper, inks, and medicinal ointments. Annual U.S. production is about 2 million kilograms. A few scale insects also produce wax.
Bee pollen and royal jelly, two other products derived from honey bees. The pollen is collected when worker bees squeeze through a special screen at the hive entrance which dislodges pollen from the hind legs. Some nutritionists regard bee pollen as a "complete" dietary supplement. It is sometimes sold in health food stores, often with astonishing claims for its medicinal or restorative powers. Royal jelly is a glandular secretion that nurse bees feed to larvae of future queens. It is rich in vitamins and proteins, and is also sold for its curative properties. It has become a major ingredient in some expensive skin care products that promise to reduce wrinkles and retard aging.
3) Raw silk: A silkworm, Bombyx mori, is the source of a unique natural fiber used to make silk cloth. This species now exists only in captivity where it is reared to maturity on a diet of mulberry leaves. As each larva spins its cocoon, it produces a continuous fiber of silk that is about 0.075 mm (3/1000 inch) thick and 900 to 1500 m (3000-5000 ft) in length. This "domestic" silk is highly valued for its light colour and lusterous finish.
Silk produced by silkworms (Bombyx mori and others) supply the raw silk in the Orient and Europe. A cocoon yields about 1,000 feet of fiber, and about 25,000 cocoons are unwound to spin one pound of silk thread.
4. Lac: The shellac of commerce is obtained from waxy secretions of lac insects (Tacchardia lacca) or scale insects (fam. Coccidae) of India, the females of which secrete lac. It is useful as a foundation of lacquers and shellacs. Laccifer lacca, a tiny scale insect that grows on soapberry and acacia trees in India and Burma, is the source of lac, a sticky resin that forms the principle ingredient of commercial shellac. Twigs bearing the scale insects are heated to extract and purify the resin. Up to 200 insects are needed for each gram of lac (90,000 per pound). Shellac, made by dissolving the lac in alcohol, was widely used as a varnish (protective coating) for floors, furniture, draperies, photographs, playing cards, and dried flower arrangements. Alkali emulsions of shellac have been molded into electric insulators, phonograph records, and dental plates. Today, lac has been largely replaced by synthetic materials, such as polyurethane and vinyl, but it is still used as a stiffening agent in the fabrication of felt hats, leather shoes, sealing waxes, and various types of inks, dyes, and polishes. Lac is the only commercial resin of animal origin.
5) Dyes: The dyes known as tannin, cochineal and crimson lac are derived from the dried bodies of certain scale insects living on cacti. Cochineal is a scarlet pigment extracted from Dactylopius coccus, a scale insect that lives on prickly pear cacti in Mexico and Central America. First used by Aztec Indians as a medicine, a textile dye, and body paint, cochineal was discovered by Spanish conquistadors under the command of Hernando Cortez (1519). The pigment was highly valued for the intensity and permanence of its colour and became a staple of trade with Europe during the 17th century. Cochineal was very expensive because of its scarcity [150 insects are needed to produce one gram of dye (70,000 per pound)], so it was used in only the finest fabrics. During the American Revolution, British soldiers, known as "red coats", wore uniforms dyed with cochineal. Today, the textile industry has largely replaced cochineal with less expensive aniline dyes, but it is still used as a colouring agent in foods, beverages, cosmetics (especially lipsticks), and art products.
6) Tannic acid is a chemical compound widely used in the leather industry (for tanning and dying) and in the manufacture of some inks. Until the mid 1800's, most of the world's supply of tannic acid was obtained from the Aleppo gall, an abnormal plant growth found on oak trees in Asia Minor. The trees produce gall tissue in response to a chemical substance secreted by tiny wasps (family Cynipidae) that infest the trees. Larvae of these gall wasps live and grow inside the galls which unwittingly provide both food and shelter for the insect invaders. Today, there is no commercial market for oak galls because tannic acid can be extracted more economically from the quebracho tree.
7) Medicinal products: Certain medicinal products like Cantheridine are also derived from blister beetles.
III) Helpful insects
1) Pollinators of flowers
‘Insect friends’ of man renders him the greatest service in pollination of flowers. Plants depend upon certain insects for cross pollination or cross fertilization which is very necessary for their fertility and the vigour. Various insects and flowers are mutually dependent since many insects feed upon their nectar and pollen grains. Chief pollinating insects are bees, wasps, beetles, ants, flies etc.
2) Scavengers
Insects feed upon waste material such as dead bodies and debris of plants and animals, thus preventing decay and obnoxious odours. Common examples of insect scavengers are silver fish (Lepisma), termites, housefly, blowfly, maggots, dung beetles, carrion beetles, fleas, cockroaches and many larvae.
3) Insects as food
Insects provide an abundant food supply for animals like frogs, lizards, snakes, fishes etc. Blue birds, meadow larks and house sparrows depend chiefly on insects. Moles, shrews, armadillos and ant eaters live wholly upon small insects. Man consumes many insects and their larvae, eggs etc., only accidently with fruits and other foods.
Insects were undoubtedly an important source of nutrition for our early human ancestors. Even today, they are still collected and eaten by people of many cultures. In Mexico, dried grasshoppers are sold in village markets. High in protein and low in fat, they may be fried or ground into meal and mixed with flour to make tortillas. Sago grubs, the larvae of a wood-boring beetle, are considered a delicacy in Papua New Guinea. The islanders boil the larvae or roast them over an open fire. Ants, bees, termites, caterpillars, water bugs, beetle larvae, flies, crickets, katydids, cicadas, and dragonfly nymphs are among a long list of edible insects that provide nutrition for the people of Australia, Africa, South America, the Middle East, and the Far East. Indeed, Americans and other descendents of western European culture appear to be unique among peoples of the world in having such a strong cultural taboo against the use of insects as food.
Natives of Amazon valley eat squab-ants. Termites form favourite food in tropics. Eggs of Curia femorata are taken in Mexico, while larvae of Goliath beetle of Africa are a fine food morsel. Greeks ground locusts in mortars and made flour of them. American Indians used to dry or smoke larger caterpillars and preserve them for later use.
A Recipe for Maggot Crispies
1/4 cup margarine
4 cups small marshmallows
3 cups crispy cereal
3 cups dry roasted maggots or mealworms
In a saucepan, melt margarine and marshmallows. Remove from heat and stir in cereal and maggots. Spread mixture in a 9x13 greased pan and allow to cool.
4) Insects in medicine
Cochineal insects contain carminic acid, coccerin, myrestin, fat and fatty acids and are used in the treatment of neuralgia and whooping cough. Blowfly larvae are used in treating decay of tissues. Cantheridin oil, made out of blister beetles, serves as hair restorer. The body extract of the cocoons of silk moth, Bombyx mori is used for eprofuse menstruation and in treating leucorrhoea and chronic diarrhoea. Bee venom has been used with some degree of success in treatment of some forms of arthritis. Bee venom has also been used in the preparation of anti venom to counteract snake bite. Honey is a natural antiseptic which prevents infection if applied to a wound. It is also applied to cure ulcers. Bee wax is used as a base for ointment.
5) Biological control
An important service that insects render is to exercise biological control over other harmful species. Insects which attack and eat up other insects are called predators. Many harmful plant eating insects are devoured by a host of predaceous insects, such as ground beetle, syrphid flies and wasps. Aphids and scale insects, which are pests of citrus and other trees, are eaten by larvae of lady bird beetles (Coccinella etc.). Predaceous insects are even reared or imported and liberated in orchards to control scale insects.
6) Insects in fine arts
Insects produce noises in various ways. Whether their sound can be called musical is disputable. In Japan, cicadas and crickets are placed in small cages, like birds, in the houses. Beautifully colour elytra and wings of some Coleoptera and Lepidoptera are used in jewellery and pictures in Central America, in crafts, in embroidery, pottery, baskets, metals, alloys etc.
2.1.5 Larval Forms In Crustacea
Crustaceans show both direct and indirect development. In direct development (e.g., Palaemon, crayfish), the adult is attained by progressive growth and differentiation of the embryo, so that the newly hatched young resembles the parents in general structure. In indirect development, there is a larval stage which differs from the adult in many features and acquires adulthood through metamorphosis. Many of the crustaceans undergo indirect development, involving a wide variety of larval forms.
Some of the important larval forms of Crustacea are:
1) Nauplius
Characteristic of the class, nauplius is the simplest and commonest type of larva, found in most marine crustaceans and a few malacostracans. When development proceeds through many larval forms, the nauplius is the earliest and the basic larva. The body is minute with 3 indistinct regions, a single median eye often referred as nauplius eye and three pairs of jointed appendages – the uniramous antennules, mainly the balancing organ; biramous antennae, principal locomotor organs and mandibles, which along with antennae may share for food collection. In branchiopods the nauplius develops straight away into the adult, but in mostly other crustaceans it may give rise to other intermediate larval forms, such as metanauplius, protozoaea, zoaea, mysis etc.
2) Metanauplius
Metanauplius is the later nauplius instar and results by the process of moulting and growth. Its body is divisible into a broad cephalothorax and an elongated abdomen, terminating into a pair of caudal forks. Besides the three pairs of nauplius appendages, it also bears the rudiments of four pairs of appendages, which are two pairs of maxillae and two pairs of maxillipedes of the adult. Some decapods, stomatopods and some notostracans (e.g., Apus) begin their life history with the free swimming metanauplius larva.
3) Protozoaea
In case of marine prawns (e.g., Penaeus), and sergestid decapods, the earliest nauplius, by growth and moulting, develops into a protozoaea larva. Its body is divisible into a broad segmented cephalothorax covered with a small carapace and a slender abdomen which is unsegmented and bear no appendages terminating in a forked telson. There is a single median nauplius eye and the appendages comprise of the anennules, antennae, mouthparts and first and second maxillipedes. The protozoaea later modifies into the zoaea.
4) Zoaea
In almost all marine decapods, except peneids and sergestids, hatching takes place at the zoaea stage (as a true crabs). Zoaea has a broad cephalothorax and a curved abdomen, which assists in swimming, is provided with a forked telson. Helmet-like carapace bears two long spines, a median dorsal and a median rostrum, two lateral spines are often met with. A pair of large stalked movable compound eyes is present. In addition to protozoaeal appendages, there appear rudiments of thoracic appendages. Biramous maxillipedes are used for swimming.
5) Cypris
In Cirripedia (e.g. Lepas, Sacculina), the nauplius larva passes into the Cypris stage. In this form the body and appendages are enclosed within a bivalved shell provided with adductor muscle as is seen in an ostracod adult, Cypris. Its modified antennules have cement glands at their bases. All other except antennae, are present. Six pairs of biramous thoracic limbs are formed. It has abdomen with 4-5 segments.
6) Mysis or Schizopod
In peneid decapods (e.g. Penaeus) and lobsters zoaea is modified into Mysis or Schizopod larva. It bears 13 pairs of appendages and resembles adult Mysis. It has 5 pairs of posterior biramous thoracic appendages. Abdomen is posterior similar to that of adult with 5 pairs of biramous pleopods and a pair of uropods and telson. In some lobsters mysis marks the beginning of the life history as the nauplius and zoaea are passed within the egg but at the same time it marks the end of the life history of a prawn.
7) Megalopa
In brachyuran decapods (true crabs), zoaea metamorphoses into the megalopa larva. It resembles, to some extent, the adult crab and possesses all 13 pairs of appendages. Abdomen bears 6 pairs of pleopods and is placed straight in line with cephalothorax. In crabs nauplius stage is passed within egg which hatches as zoaea. It then by moulting forms megalopa to be metamorphosed into adult. In hermit crab, the glaucothoe corresponds to a megalopa with symmetrical abdomen and swimming pleopods.
8) Phyllosoma
Larva of Palinurus, the spiny crab or rock lobster, is called Phyllosoma or glass crab. It is a modified mysis stage. It is remarkably large, flattened, leaf like, delicate and glassy. Body is distinguished into head, a transparent thorax and abdomen. Eyes are compound and stalked. Out of six pairs of thoracic appendages, the first or maxillipedes are rudimentary, second are uniramous, third well formed biramous succeeded by rest 3 pair of long biramous legs. A segmented but limbless abdomen is present. Before reaching an adult stage, it undergoes several moulting.
9) Alima
It is modified form of zoaea found in some malacostracan (e.g. Squilla) which hatches from egg. It is a pelagic form with glassy transparency having a slender body. It bears short and broad carapace. It has all the cephalic appendages but only first two thoracic ones. A six segmented abdomen with 4 or 5 pairs of pleopods is present. It differs from zoaea, in having well formed second maxillipedes and the armature of the telson.
2.2 MOLLUSCA
2.2.1 ECONOMIC IMPORTANCE OF MOLLUSCS
Molluscs are of major interest to man as about 10,000 species are of economic importance. Mostly they are beneficial to man although there are some molluscs which are indirectly harmful.
I) Beneficial molluscs
1) As food:
Along the world's miles of coastline, man has always had a readily available food source -high in protein and trace minerals, because of the many kinds of molluscs to be found there. Mussel and oyster beds, clam-flats and other abundant shellfish have always provided an easy source of food.
Today, fisheries in Europe, Japan and the US alone produce over 1 billion pounds of oyster meat each year. Abalone, a great delicacy, can fetch up to three hundred dollars per pound. Chitons formed the main food of Red Indians. The gastropods are consumed by numerous predators, chiefly fish, birds and mammals. The large land snail Helix pomatia, large foot of Haliotis and apple snail (Pila) form common food in New York, California and South India, respectively. Oysters, scallops, marine mussels and clams have been often used for food. Oysters are served fried, the hard shell marine clam, Venus mercenaria as whole on half shelled or cooked in chowder, Mya arenaria, a soft shelled clam, is steamed in shells and served with butter. Mytilus edulis is used in chowder, and adductor muscle of Pecten are served in flour and fried. Pelecypods also furnish food for star fish, boring sponges, drilling snails, some marine leeches, fish and shore birds. Cephalopods also form food for other animals like marine mammals and large fish.
- Trade Goods:
Shell currency has been around for over 4,000 years and was, in it's heyday, the most widely used currency in the world. Even today, there still exist minor currencies based on certain shells. Cowrie shells (Cypraea annulus L., and C. moneta L.),
Collected loose in bags or strung into strands, were the earliest forms of currency used in many countries. The Chinese, so far as we know, were the first people to use cowries as currency. Here, cowries have been found in prehistoric Stone Age sites. Examples of other country's native money-strands are the diwara in New Guinea, rongo in the Melanesian islands and sapisapi in Africa. The image of the cowrie as a type of currency was so strong that the first oval metal coin minted in the Greek colony of Lydia around 670 B.C. was modeled after that shell. By the eighteenth century, approximately 400 million cowries were being traded per year mostly for the purchase of black slaves. By the middle of the nineteenth century, it could take up to 100,000 cowries just to buy a young wife. Inflation, it seems, was the main demise of the cowrie currency.
Red Indian tribes of America used the common Dentalium indianorum as sawampum or money. Value of shells varied lengthwise. Gastropodan shells were source of money for various native races, including Vampum of American Indians. Shell of gastropods made media of barter in Africa and other countries. American Oyster, Crassostria viginica, is commercially cultivated and harvested and provides millions of dollars to the industry. Squids, cuttle fish and devil fish earn money as they are sold in market for food in China, Japan, India and Italy.
3) As bait:
Many gastropods are very useful to man, as bait for catching fish. Squids make excellent bait for marine fishes especially cod in United States. Small Octopus is used as bait by the line fishermen of Palk Bay.
4) Ornamentation:
Scaphopod, Dentalium indianorum, tooth shells are valued as ornaments. Tools, utensils and objects of delight have been formed from gastropodan shells. Some marine snails of South Pacific (turban shells) have a calcareous operculum so rounded and coloured as to resemble a vertebrate eye. These opercula known as “cat’s eye” are sought as curios. Nautilus shell is much used for decoration, art and for many other useful purposes.
5) Useful dyes and ink:
Some gastropods like Nucella (Purpura) and Murex are sources of Tyrian purple from their juices. Dye for royal purple in Biblical literature originally came from a gland of the snail, Murex truncutus. Secretion is colourless but becomes a beautiful purple by exposure to the air. Contents of ink-sac of cuttle fish provides a rich brown pigment called “sepia”, used by the artists. Now-a-days a certain brown finish of photograph is termed as sepia finish.
6) Buttons and pearls:
Gastropodan shells are used to manufacture buttons and other articles. Shells of certain bivalves have been used for mother-of-pearl layer also for buttons, knife handles etc. Buttons are made by hand by cutting shells of freshwater bivalves and some marine clams. Pearls are made by clam and pearl oysters themselves and are among the most beautiful and valuable of our jewels.
- As buttons and fasteners Abalone shells (Haliotis), especially the famous Paua shell (Haliotis iris Martyn) from New Zealand, were once extremely popular for buttons - until plastic took over!!
7. Tools:
From prehistoric times, man has used shells for tools. This practice has been born out by archaeological findings in ancient sites and still carries on even today. Some examples of these shell tools are Household dishes, cooking pots and utensils: cutlery, scoops, spatulas, etc. were often made from bivalves and larger gastropods. Storage containers for such things as perfumes, ointments and medicines were made from some of the larger bivalves and univalves such as the Nautilus. Oil lamps made from shells are a frequent find throughout the Middle East. There are examples of these made from bear paw shells (Hippopus hippopus Linne) and the spider conch (Lambis spp). They work by holding oil while the wick floats on the surface.
8. Jewellery:
Shells have been used to make Jewelry for thousands of years - especially valued for this is the exquisitely iridescent (i.e., containing all colors of the rainbow!) interior of Abalone (Haliotis spp) shells, and the shiny "mother of pearl" interior of oysters and several other bivalves.
9. Religion:
Shells have played a central role in religion from prehistoric times on. Dominating early religious practices, cowry shells (Cypraea) had powerful symbolism (basically sexual, for they were first and foremost a female symbol) and this was renewed in the religions of the great civilizations that followed. The presence of shells in prehistoric burial places indicates that their symbolic power was believed to continue beyond life. Shells in some cultures even today are used as amulets, good luck charms, and as symbols for love, fertility and life eternal.
Some examples of some these religious practices are:
- Africa: Shells fetishes were often used in worship. Ceremonial garbs are many times decorated with shells and were used in some religious ceremonies.
(Note: a "fetish" an object which is treated with reverence and respect because it is either thought to have special powers, or is where a god or spirit lives or is present in some special manner)
- North American Indians also made fetishes of shells. The Canadian Ojibwa tribe maintained a Grand Medicine Society in which the sacred emblem was a shell.
Long before our modern day communication systems, man found that trumpets made from shells produced a sound that carried for many miles. By using as series of trumpet blasts, messengers were able to communicate fairly detailed messages from village to village, tribe to tribe. In many countries shells have also been tied together or had such things as sand or beads sealed inside them so that they became as sort of rattle to accompany song and dance.
Some ways in which shells were or still are used are:
· as a summons to religious ceremonies as well as often playing a role in the ceremony itself.
· as a daily call to prayers. Shinto priests in Japan still use the Triton Trumpet shell (Charonia tritonis L.) for this today.
- as a summons to call warriors to battle and to ring out triumphs in battle.
- Art and Architecture:
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Man has long been inspired by the graceful symmetry and beauty of shells. Archaeological diggings at many ancient sites have produced shells and artefacts in the design of shells. Phoenicians, Greeks, and Romans used the shell's shape as part of their building design and decor. Shells and shell motifs have often been incorporated into man's homes and public buildings. Architecture has been profoundly influenced by the symmetry of molluscs. Many great artists were so inspired by the beauty, diversity and design of the shell, that they incorporated them into their masterpieces. Shell cameos are made mostly from snails notably that of Cypraea tigris and Cases tuberose. Nautilus shell is commonly used in art. It is pretty object thrown ashore during monsoon storms on the Indian coasts. Here are a few examples of shell artistry, famous artists and architectures:
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- Botticelli's Birth of Venus has Venus rising from the foam in a scallop shell. In the ancient world of the Mediterranean, this theme of Aphrodite's (Venus's) arising birth from the shell repeats itself in figurines and wall paintings.
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- Pierodella Francesca incorporated the scallop shell Pecten jacobaeus L. into his art.
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11. As medicine:
The internal calcareous shell of Sepia is used as medicine as well as for other purposes.
Shell amulets were once thought to ward off ill health, infertility or bad luck. Shells have also been ground up for use in potions and for various medicinal uses throughout history. Today the shell, its living flesh and by products are being studied and used in many areas of medicine. Some examples:
- Paolin, a drug made from abalone juice, is an effective inhibitor of penicillin- resistant strains of bacteria, such as Staphylococcus aureus, Streptococcus pyogenes, Salmonella typhus and parathyphoid A & B bacteria.
- An extract from the hard clam or "Quahog" (Mercenaria mercenaria L.) is a strong growth inhibitor of cancers in mice. It is called mercenine, after the clam's scientific name Ground and processed oyster shells are used as a calcium supplements both for humans and animals.
Oyster juice has been found to have anti-viral properties, and may be made into a drug eventually. The cement of the Carrier Shells (Xenophoridae) is being studied for use as a possible cement for bone fractures.
12) Bioindicators :
Bivalve molluscs are used as bioindicators to monitor the health of aquatic environments in both fresh water and the marine environments. Their population status or structure, physiology, behaviour or the level of contamination with elements or compounds can indicate the state of contamination status of the ecosystem.
II) Harmful molluscs
1) Herbivores- as pests:
A few species of snails and slugs are serious agricultural pests; in addition, accidental or deliberate introduction of various snail species into new territory has resulted in serious damage to some natural ecosystems. Some gastropods like land slugs and snails cause damage to gardens, orchards, green houses and mushroom beds by feeding upon the succulent parts of seedlings and mature plants. They do not spare the vegetable crops, flowering plants and other decorative plants. Certain pelecypods burrow or bore into wood and stone.
2) Carnivores:
Some gastropods are ferocious predators (Natica, Buccinum, Murex, Urosalpinx), that bore into and fed on other molluscs. Marine snail Urosalprinx causes serious losses to oyster industry. It is known as ‘oyster drill’. Cephalopods are all predaceous and carnivorous molluscs, devouring great number of fish, crustaceans and other molluscs and are much destructive to fisheries.
All species of cone snails are venomous and can sting when handled, although many species are too small to pose much of a risk to humans. These carnivorous gastropods feed on marine invertebrates (and in the case of larger species, on fish). Their venom is based on a huge array of toxins, some fast-acting and others slower but deadlier; they can afford to do this because their toxins require less time and energy to be produced compared with those of snakes or spiders. Many painful stings have been reported, and a few fatalities, although some of the reported fatalities may be exaggerations. Only a few larger species of cone snails which can capture and kill fish are likely to be seriously dangerous to humans. The effects of individual cone-shell toxins on victims' nervous systems are so precise as to be useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them. All octopuses are venomous, but only a few species pose a significant threat to humans. Blue-ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked, but their venom kills 25% of human victims
The traditional belief that a giant clam can trap the leg of a person between its valves, thus causing drowning, is a myth.
3) Parasites:
Members of the families Pyramidellidae (e.g. Brachystomia) and Eulimidae of gastropoda are ectoparasites and suck blood from bivalve molluscs, polychaetes and echinoderms. Stylifer (family Styliferidae) is an endoparasite in the wall of echinoderms. Among the pelecypod parasites, Entovalva lives within the gut of a sea cucumber and absorbs pre-digested food of the host.
4) Intermediate hosts:
Snails are of considerable importance from a medical point of view as many of them serve as intermediate hosts for parasitic flat worms, such as Schistosoma and Fasciola. Schistosoma cause disease known as Schistosomiasis (also known as bilharzia, bilharziosis or snail fever). It is transmitted to humans via water snail hosts, and affects about 200 million people.
5) Commensals:
There are commensal pelecypods which include – Phylyctaenachlamys living in burrows of a shrimp of Great Barrier Reef, Lepton in burrows of shrimps and Polychaets, Mediolaria in the test of sea-squirts.
2.2.2 SHELL AND FOOT MODIFICATION IN MOLLUSCA
2.2.2. A SHELL IN MOLLUSCA
Shells are lovely natural objects, equals in beauty to any flower or butterfly, they are more than just pretty baubles found on beaches. They are the exterior skeletons (exoskeletons) of a group of animals called mollusks. The word "mollusk" means "soft-bodied;" an exterior skeleton is very important to these creatures, providing them with shape and rigidity, and also with protection, and sometimes camouflage, from predators.
Molluscs are classified into major groupings according to the characteristics of their shells. Snails (Gastropoda) have a single shell which spirals outward and to one side as it grows. Most Cephalopoda (octopi and squid) have no shell, but the Chambered Nautilus of that group has a shell. This shell does coil, but it coils flatly, in a single plane. Tusk shells (Scaphopoda) also have a single shell, but it does not coil at all; it grows in a narrow and very slightly curved cone shape. Bivalves (Bivalvia), including oysters, clams, scallops and mussels, have two parts to their shells that enclose their tender bodies like the two halves of a hinged box. Chitons (Polyplacophora) are little armored tanks, with a row of eight overlapping plates protecting them. The Neopilina (Monoplacophora), are deep-sea "living fossils;" they have a single shell which hardly coils at all, but fits over their bodies like a protective cup. (Some gastropods (the limpets) have shells like this too, but their body structure is very different.) Last are the deepsea worms-like Aplaco
The molluscs are characterised by having as a rule, a protective exoskeleton in the form of a shell. Shell is usually external, sometimes internal. It is derived from the mantle of the veliger larva. In the adult the shell is represented by the smallest whorl at the apex of the shell. It may be univalve, bivalved, cone shaped or spirally coiled and sometimes in a linear row of eight valves.
A large variety of shells are found in molluscs. There are six major classes of Molluscs that have shells:
1. Shell in Gastropoda- 1 shell, twisted around a central axis
The typical shell of the gastropod is the familiar conical spire composed of tubular whorls. This shell, which is created, maintained, colour and modified by the mantle, contains the visceral mass of the animal: i.e., all its internal organs. Starting at the apex, the smallest and oldest part of the shell, whorls get successively larger and are coiled about a central axis called the columella, which may be open or closed. The largest whorl terminates at the aperture or opening where the head and foot of the animal protrude.
A shell may be spirally clockwise (right-handed shell) or anti clockwise (left-handed shell). When a shell is held so that the apex (top) is up and the aperture facing the person, those with the aperture facing to the right is right-handed or dextral and those that open to the left are left-handed or sinistral. Both sinistral and dextral shell can be found amongst members of the some species. A snail shell is coiled spirally, but the spire is clearly visible on one side of the body. Even in those snail species, whose shells do not reveal the spire on first sight, after some examination it will be found that there is a difference between the spire apex (tip) on one side of the shell and the hollow of the umbilicus (shell navel) on the other.
As the snails' shells are coiled asymmetrically, right-handed (dextral) and left-handed (sinistral) shells can (and must) be distinguished. When facing a snail shell in the apertural view, the aperture pointing to the bottom, in most species one can easily see that the shell whorls run to the right. On the other hand, there are also snail families, such as the door snails (Clausiliidae), among which most species are left-handed, with some exceptions. And there are also families like the Bulins (Enidae), among which there are right-handed and left-handed species in about equal numbers. The coiling direction of snail shells, however, is mostly species specific, which means that a shell's coiling direction can be used as a means of identifying the species it belongs to.
The first shell whorls lay down by the larval gastropod (i.e., while it is in its egg), are called the protoconch (proto = before, conch is shell). It is represented by the smallest few whorls at the apex of the shell, and is usually smooth, and lacks many of the characteristics of the adult shell, often being colourless, or of a different colour from the rest of the shell.
A typical gastropod shell is composed of three layers; the outer periostracum, the middle prismatic layer (Ostracum) and an inner nacreous layer (Hypostracum). The periostracum is thin and composed of a horny organic (made out of protein, actually!) material called conchiolin, which is semi-transparent, being a brown colour: the thicker the periostracum, the darker the colour, and the more the shell underneath is both protected from sand grains and other abrasive elements of the animal's environment, as well as from acidic water, which some of the hardier gastropods and bivalves can survive in. The two inner layers are composed of calcium carbonate. In the middle layer (which we normally think of as the outside of the shell, since it contains the colours and patterns, the calcium carbonate is laid down as vertical crystals. In the thin, inner, nacreous layer, the calcium carbonate is laid down in thin horizontal sheet. Reserve calcium carbonate is stored in certain cells of the digestive glad and is used for shell repair or to add new growth thus enlarging the shell for the growing animal. Molluscs can only form shells when they can extract CaCO3 (calcium carbonate) from the water, and keep it from being dissolved again. Thus, if a lake or stream is acidic, or the soil is acidic (as in Coniferous woodlands), shells, and therefore the animals that make them to protect and support themselves, cannot survive. Also, below a certain depth in the ocean (which varies with temperature and mineral content, calcium carbonate cannot be deposited, since the water is under-saturated with Ca CO3. This is called the calcium carbonate compensation depth, and no shell-bearing molluscs can survive below this level. To summarize, shells can only be formed in fresh waters that are non-acidic, and in the ocean at depths above the level where the water becomes under saturated with Calcium Carbonate.
Gastropods show an infinite variety of colours, patterns, shapes and sculpturing of their shell (which is why people collect shells, as opposed to the entire mollusc!!). In some gastropods, the shell is only conspicuously coiled in the juvenile stages. The coiled nature disappears with growth, and the adult shell represents a single large expanded whorl. Examples of this are found amongst the abalones (Haliotis), limpets (several families, including Lottiidae and Acmaeidae) and slipper shells (the familiar Crepidula and Capulus). (The limpets became secondarily symmetrical during in their evolution.)
In the family of Vermetidae (the worm shells), the larval and juvenile shells are typical, but as the animal grows older the whorls become completely separate. The adults look much like a corkscrew, and sometimes don't coil at all, forming a tangled mass of tubes called a colony.Amongst many gastropods, the shell has become much reduced or is absent completely. In other cases the foot and mantle are very large and the mantle has reflexes backwards over the shell so that it becomes totally covered. These animals are no longer able to pull their bodies completely into their shells. The pulmonates show varying degrees of shell reduction and loss, culminating in the slugs (land and sea), which have no shells at all.
The opisthobranchs have a much reduced shell which is closely related to the degree of detorsion they have undergone. A few have well developed shells, such as the bubbles, but most have a much-reduced shell that is often covered by the mantle as is found in the sea hares.
Gastropods are able to withdraw into their shells by means of a retractor muscle. This muscle, called the columellar muscle, arises from the foot and it is inserted into the columella. The most ancient gastropods, the abalones and limpets have two of these muscles but in the more modern gastropods the left muscle has disappeared.
2. Shell in Bivalvia-
Class Bivalvia- Peicipoda: Latin; bi=two - two plates (Two halves to the shell) Pele=hatchet pod=foot hatchet foot (shape) The Pelecypoda, Bivalva or Lamellibranchia (Latin for leaf-gill) (the only class with three names!!) is comprised. - bivalves: 2 shells hinged dorsally at the umbo.
The typical bivalve shell consists of two similar, convex and oval or elongate valves. These valves are attached and articulate with each other. The shell is made up of three layers: The periostracum or thin outer layer that is made of horny, organic material called conchiolin, the prismatic or thick middle layer that is made up of calcium carbonate crystals arranged in vertically, and the nacreous or thinner inner layer that is composed of thin horizontally arranged calcium carbonate crystals.
Dorsally, the shell has a protrusion called the umbo, which rises above the articulation (most commonly called teeth). The umbo is the oldest part of the shell. The concentric lines found around the umbo are growth lines, which are usually seasonal, making it a lot easier to tell the age of a bivalve than a gastropod! The two valves are attached by an elastic band of cartilage-like material called the hinge ligament, which is made up of conchiolin - the same as the periostracum. The hinge ligament is designed to hold both halves of the shell together. The main muscle of a bivalve is called the adductor muscle, and is also used to hold the shell halves together. When the bivalve’s adductor muscles relax, the ligament causes the valves to open. Most species are also equipped with locking teeth or sockets beneath the ligament to prevent lateral slippage. The valves are pulled together through the action of the two strong, adductor muscles. They are antagonistic to the hinge ligament, as just explained. On the inside of the shell is a scar that marks where these muscles attach.
In most bivalves, the valves are similar in structure and size; however, in a few families such as the oysters and jingle shells the upper or left valve is always larger than the right valve. For those bivalves that attach to a substratum, they always do this by their right valve, which is always the smaller of the valves, if there is a difference between them.
The bivalve shell exhibits a great variety of shapes, sizes, surface sculpturing and colors. In size they range from a few millimeters (<2mm in the Sphaeriidae) to over four feet (1.3+ meters in the Tridacna gigas). They come in all colors and color combinations, and range from smooth as glass to having long sharp spines in their sculpturing (e.g.: the Spondylidae, or "spiny oysters".)
The mantle is attached to the shell, in a semicircular line just inside the shell edge, by means of the inner lobes' circular muscle. This attachment leaves a visible scar on the inside of the shell known as the "pallial line". The pattern of the pallial lines and adductor muscle scars is extremely useful in identifying very similar species, when you only have the shell.
Example-Shell of Mytilus
Shell is equivalved and wedge-shaped being pointed in front and rounded behind. Two shell valves are united antero-dorsally by a hinge ligament. Hinge ligament is made of conchiolin and is brown, tough, elastic and non calcareous. Umbo, a whitish knob like swelling in each valve lies anteriorly. Umbo is the thickest and oldest portion of the shell. Lines or rings of growth run around umbo as centre and run parallel to the free margin of the shell.
3. Shell in Polyplacophora – The polyplacophores, more commonly known as chitons, live in the water and are covered by 8 protective plates.
Example - Shell of Chiton-: 8 valves embedded in a girdle (tough mantle region)
Shell forms the most characteristic feature of chitons. It is made of eight transverse, overlapping, calcareous plates or valves, arranged in a longitudinal row. It forms the solid armour covering the dorsal surface. Name of order Polyplacophora is derived from this nature of shell. Shell plates are moveable upon one another allow the animal to roll up like a wood louse. First (cephalic) and eighth (anal) shell plates are hemispherical in shape, while others, called intermediate shell plates are somewhat rectangular and often keeled mid-dorsally. Each shell plate is made up of two distinct layers. Tegmentum or upper layer consists of an organic conchiolin matrix and lower thicker and denser layer called articulamentum consists of calcium carbonate only.
4. Cephalopoda – The Cephalopod class includes the octopus, squid, and the chambered nautilus. The chambered nautilus is the only cephalopod with a shell. Nautilus has 1 shell, squid with internal shell while octopus has no shell.
The pelagic Cephalopod family Argonautidae, commonly know as the paper nautilus, have a remarkable adaptation for egg deposition. The two dorsal arms of the female are greatly expanded at their tip to form a membrane. The expanded portion of each arm secretes one half of a beautiful calcareous bivalved shell. She then deposits her eggs directly into this case. The shell acts as both a brood chamber and a retreat for the female. The posterior of the female usually remains in this shell. The male, which is much smaller than the female, does not have such a shell.
Example -Shell of Sepia
Shell of cuttle fish or Sepia is internal and lies embedded in upper side, completely enclosed in a sac of mantle. It is secreted by its epithelial lining. It is flat, broad and oval in shape, represented by phragmocone with a broader and rounded oral end, called pro-ostracum and a narrow, pointed aboral end called rostrum, projecting into a spine. Shell is dead and composed of calcareous rather than horny matter. Hard and resistant shell provides rigidity to trunk, like an endo-skeleton. It is commonly spoken as cuttle bone and is a familiar object of seashore.
Example- Shell of Dentalium
Shell is external in Dentalium. It is cylindrical tubular, slightly curved and tapering, which is open at both ends. It is shaped like a trumpet or elephant’s tusk, hence often called “tusk shells”. During life, shell is buried obliquely in mud with wider anterior end lying deepest and narrow posterior end of apex projecting above the surface of mud. The scaphopod shell differs from that of other molluscs in being unchambered and open at both ends.
6. Shell in Monoplacophora - - Their simple shells are less complicated and evolutionarily advanced than other molluscs. shell-less aplacophorans have calcareous spines
· Functions of shells and shape -
- Shell is used for physical defence against predators. It also acts as a type of exoskeleton, to which muscles can attach and work off of.
- Molluscs' shapes are a product of heredity combined with habitat and life style for the most part. Shell shapes have evolved to make their lives easier.
- A snail that burrows through sand needs a shell that will move through wet sand easily -- best is a smooth and slender and gradually tapering shell, narrow end at the front, with no impeding projections, but molluscs have worked out many variations on this theme.
- A shell that needs lots of camouflage may have evolved a spiny or irregular surface which will catch and hold all sorts of camouflaging encrusting organisms.
v. Spines are also helpful for discouraging predators and, in some arrangements, for life on mud...broad weight distribution.
· Biggest marine shells
The biggest marine snail is Syrinx aruanus, the Australian Trumpet, at 77.2 cm. (over 30"). The biggest American marine snail, the Horse Conch, Pleuroploca gigantea, is just over 2'.
2.2.2. B.FOOT IN MOLLUSCA
The molluscs are characterised by presence of large massive foot. The foot is a ventral muscular distinct from the rest of the body. The foot in mollusca shows different modifications depending upon the mode of life and locomotion. The chief function of foot is locomotion. Being an important organ, it has undergone changes in shape and form for different habitats. Various shapes of foot are the basis of classification of Mollusca. Foot in different classes of Mollusca is described as follows:
i. Foot In Monoplacophora
The foot in its vestigial form is found in the primitive molluscs like Neopilina, Neomenia and Chaetoderma of order Aplacophora; in such cases, the foot is represented by a mid-ventral longitudinal groove. In the order Polyplacophora (Chiton), the foot is represented by a broad muscular wave from the anterior to posterior end, which helps the animal in the creeping movement. It also helps the animal to adhere to any object firmly.
Example- Foot in Chitons have a broad flat foot, which occupies greater part of the body organs. It is modified for creeping on the object. The foot is large and muscular, present at ventral side of the body. It serves both for locomotion and adhesion. It also serves as a sucker to adhere firmly to rocks, when at rest. The foot secretes a small amount of mucous and propulsion is accomplished entirely by muscular contraction. Both the foot and the girdle affect adhesion. Ordinarily, adhesion is accomplished just by means of the foot; however, when disturbed, the girdle clamps down on the hard substratum and the inner margin is raised. This creates a vacuum that enables the Chiton to grip the surface with great tenacity.
ii. Foot In Scaphopoda
In the class Scaphopoda the foot is narrow, cylindrical and directed in forward direction. It is capable of being protruded through the oral shell aperture and used as a digging organ. The lower free end of the foot is conical and trilobed, carrying on each lateral side a wing like fold which is comparable to an epipodium. The muscular activity of body moves the foot in forward and backward direction.
Example- Foot of Dentalium
Foot when fully extended is engorged with blood and in Antails entails (previously known as Dentalium entails) erectile side lobe are distended to help anchor foot in sand. Shell is then pulled down to foot. Burrowing is accomplished by the muscular tongue.
iii. Foot in Gastropoda
Gastropoda (gaster, belly; podos, foot) means belly footed animals. The foot is a thick muscular fold, lying generally on ventral surface, beneath the head and digestive tract. The foot is generally divided into three parts. These are -
i. Propodium, the anterior part of foot and are well developed in forms, which crawl on the wet sand. Example: Sigaretus, Natica. By foot the animal pushes sand away on both sides of its path.
ii. Mesopodium is the middle large portion of foot.
iii. Metapodium is the posterior part of the foot which bears the calcareous operculum. Gastropods creep upon the broad, flat, ventral foot by a process of body deformation. Small waves of contraction sweep along the length of the foot; one wave closely follows the wave. Area of foot in contracted region is lifted; at relaxation, it is replaced on the substratum, a little in front of point from which it was raised. During each wave of contraction, a small section of foot performs a little step. The summation of all these little steps results in gliding motion. Mucous produced by a large gland in the foot lubricates substratum. In some gastropods, waves sweep from the back to front (direct waves). In other species, waves move from front to back (retrograde waves), opposite to direction, in which animal is moving. Waves may extend all the way across the foot (monotaxic), or those on one side of foot may alternate with those on other side (ditaxic). Most common mode of muscular pedal creeping in gastropods is by ditaxic retrograde waves.
Modification of foot in gastropods
Due to different condition of life:
i. In sedentary forms like Vermetus, the foot has been reduced to a simple small disc.
ii. In clinging forms like Bat hysciadium, the ventral surface of foot functions as a sucker to cling to any object.
iii. In parasitic forms like Stylifer, the foot has reduced and is represented only by a small ventral appendage.
iv. In crawling forms like Bulia, the foot has extended on all the sides of body and it slides over sand in order to progress forward.
Structural changes in the form of appendages:
i) The epipodia is a distinct fold, extending from head to posterior end of foot as in Haliotis. The epipodia is well developed and gives out lateral tentacles as found in Monodonta. Two anterior angles of foot are prolonged as tentacles as found in Valvata. In Atlanta and Rostellaria, the posterior part of foot is separated from rest to form a lobe bearing the operculum.
ii) The Parapodia are lateral broad, lobes of sole of foot. It is generally used in swimming as found in Aplysia. In Notarchus, the extensions are united above the body forming a muscular sac. It is open in front and closed behind and at sides. By forcing water through the anterior aperture, the animal moves using sac as organ of locomotion.
iv. Foot in Bivalvia (Pelecypoda or Lamellibranchia)
Most bivalves burrow within the sediment. Burrowing involves foot, shell and siphons. Foot tends downwards in a probing motion and then expands to form anchor. Siphons close to prevent discharge of water. Adductor muscles soon close the valves ejecting water from ventral margin. This is immediately followed by the contraction of foot retractor muscles, pulling the animal downward towards the anchored foot. Lastly, the adductor muscles relax and the ligament opens the valves. Some bivalves bore into wood or even solid rock. Ship worms burrow into wood using their shell as a file. Lithophaga secretes a weak acid from its mantle, erodes the weakened rock by rotating its shell, e.g. Telina, Cardium, Mya, Pholas, Ensis and Axinus.
Example- Foot of Mytilus-The foot of Mytilus is a small, conical in shape attached with large number of prehensile thread like tentacles. The foot is present at ventral side, and along the margins of foot there are byssal threads. These threads help during swimming movement. They are also helpful during attachment to the object substratum. These threads are sensory in nature.
v. Foot in Cephalopoda
In the class Cephalopoda the foot does not resemble with typical molluscan foot, but modified partly into 8 to 10 cephalic arms for seizing prey and swimming activity. Cephalopods crawl, or swim with the involvement of mantle. Water enters in the mantle cavity by the relaxation of circular muscle and simultaneous expansion of the mantle. Water is forced out through the funnel during contraction of the mantle giving the animal the power to move. Locomotion occurs by rapid undulation of the outer edges of fins in squids.
Example-Foot of Sepia- Sepia shows 5 pairs of arms bearing stalked, suckers provided with horny rims. The fourth pair of arm, called as tentacles, is long, prehensile into basal pits and bearing suckers only at their free club shaped ends.
Foot in Octopus-It is bottom crawler, swim rapidly by jet propulsion in disturbed situations. The finned octopods are provided with paddle- shaped fins that are used in hovering and slow swimming.
2.2.3. TORSION AND DETORSION IN MOLLUSCS
Torsion in Molluscs:
Torsion or twisting is a process, during larval development of gastropods, which rotates the viscera-pallium anti-clockwise through 1800 from its initial position, so that mantle cavity, with its pallial complex, is brought in front of the body, in adult.
Site of torsion
In larval gastropods, only visceral mass undergoes rotation through1800, whereas head and foot remain fixed. Actual site of torsion is neck, behind the head-foot, through which oesophagus, rectum, aorta, visceral nerve loop and shell muscles pass. Thus, actual twisting involves the neck tissue and structures within it.
How torsion occurs?
Torsion is not merely an evolutionary hypothesis. Its occurrence can be seen in the embryogeny of living gastropods. A ventral flexure of the body results in looping of alimentary canal and approximation of mouth and anus. Shell and visceral mass, originally saucer-shaped, become first cone-shaped and later spirally coiled. Shell lies dorsally and forms a coil on the anterior side; such a shell is called exogastric. Ventral flexure is followed by a lateral torsion, so that dorsal or exogastric shell becomes ventral or endogastric. Generally, growth of the right side becomes retarded so that mantle cavity and pallial complex gradually pass round to right side, and so to the anterior side, on account of greater growth of the visceral sac towards the left.
Thomson (1958) after careful study recognises five ways in which torsion can be brought about –
1) Complete or 1800 rotation, achieved by muscles contraction alone, is known only for Acmaea (Archaeogastropoda).
2) 1800 rotation achieved in two stages, first 900 movements by contraction of larval retractor muscles and later a slower 900 rotation by differential growth. It is the commonest mechanism which is known today, e.g., Haliotes, Patella.
3) 1800 rotation by differential growth processes alone, e.g. Vivapara.
4) Rotation by differential growth processes, with anus coming to a position appropriate to adult state, e.g. Aplysia.
5) Torsion no longer recognisable as a movement of viscera-pallium, the organs in post-torsional position from their first appearance, e.g. Adalaria.
Detorsion in Molluscs
Changes occurring in torsion are to a certain extent reversible. This reversion is known as detorsion and it is very characteristic of the whole group of the Euthyneura. As a result, pallial complex travels back towards the posterior end along the right side, ctenidia point backwards, auricles move behind the ventricle, and the visceral loop becomes untwisted and symmetrical. In this way, a secondary external symmetry is re-established. Torsion must be disadvantageous to adult snails, as many of them have undergone detorsion processes. Various degrees of detorsion are met within the Euthyneura.
The phenomenon of detorsion can thus be elaborated as follows –
1) In some cases the right ctenidium (originally left) and the osphradium are absent.
2) In Eolis, there is veliger larva with a coiled visceral hump that undergoes torsion but adults do not show any sign and the pallial complex is posteriorly placed in adult. Naturally, detorsion must have occurred during the course of further development.
3) In Pulmonata, the pallial complex is shifted but there is no chiastoneury as a result of shortening of visceral commissures. The pleurovisceral mass and so the chiastoneury is secondarily lost.
2.2.4. LARVAL FORMS IN MOLLUSCS
There are different types of molluscan larvae according to the importance of the pelagic phase and amount of planktonic food taken. G.Thorson (1950) recognised three ecological types of larvae in mollusca.
1) Planktotrophic larvae with long larval life
Such larvae have larval life of two or three months e.g. Lamellibranchs, Prosobranchs. They are capable of wide distribution. They are usually found in tropical, subtropical and a few in high Arctic seas. Such molluscan veligers are all ciliary feeders. The large velar cilia collect particles which are thrown on to a tract at the base of the velum leading to a mouth. Coarse or unsuitable particles are removed by rejectory tracts upon the foot.
2) Planktotrophic larvae with short swimming life
Such larvae have larval life of not more than a week in the plankton e.g. Nudibranch larvae, Gibbula cineraria, Hydrobia ulva, Turitella communis and Bela trevelyana. The velum never elaborates. Planktonic feeding is of secondary importance. Distribution is the main object of their life. There is little growth between hatching and settling. Being less dependent on food, they are surprisingly adaptable to unfavourable conditions, and serve mainly for dispersal.
3) Yolk larvae
Such larvae take no food in plankton. They are lecithotrophic, as they hatch from very yolky eggs and develop into large, ‘yolky larvae’. They swim little and are passively carried about in the plankton. Gastropods show only few examples of yolk larvae. Yolk larvae are normally found in Amphineura, Scaphopoda and protobranchiate Lamellibranchia. In Chiton, the yolk larvae are modified egg-shaped trochophores with a broad cilliary ring. They spend six hours to few days in the plankton. Neomenia has three such rings, Dentalium has four.
Beside above mentioned ecological larval forms in mollusca Glochidium, veliger, trocophore larvae are present. In Pelecypoda development is indirect Glochidium larva is present. In Gastropoda developement indirect. Velliger / trochophore larvae present. In Cephalopoda development is direct and Larva absent. In Monoplachophora and Polypachophora larvae absent.
Trochophores exist as a larval form within the trochozoan phyla, which include the entoprocts, mollusks, annelids, echiurans, sipunculans and nemerteans. Trochophore larvae are often planktotrophic; that is, they feed on plankton. Trochophores are hatched from eggs. The stadium of a trochophore larva lasts for a few hours and then it changes into another free-swimming veliger larva (in some gastropods and in some bivalves).
The veliger is the characteristic larva of the gastropod, bivalve and scaphopod taxonomic classes that is produced following either the embryonic or trochophore larval stage of development. This stage in the life history of these groups is a free living, planktonic organism that potentially enhances dispersal to new regions far removed from the adult mollusks that produced the larvae. The general structure of the veliger includes a shell that surrounds the visceral organs of the larva (e.g., digestive tract, much of the nervous system, excretory organs) and a ciliated velum that extends beyond the shell as a single or multi-lobed structure that is used for both swimming and particulate food collection. The larva may have or may develop a foot that will be used by the newly settled veliger as it moves about and searches for an appropriate place to metamorphose and subsequently by the juvenile for benthic locomotion. The velum and foot of the veliger may be retracted into the shell for protection of these structures from either predatory or mechanical damage.
Veligers hatch from egg capsules or develop from an earlier trochophore larval stage.
Veliger of gastropods
The veliger is the second larval stage in the development of gastropods, following the earlier, trochophore, stage. In many species, including virtually all pulmonates, the veliger stage is passed before the eggs hatch, being an embryo within the egg, rather than a free-living larva. When it is free-living, the veliger is exclusively aquatic. Free-living veliger larvae are typically filter-feeders, but some retain yolk from the egg within their bodies, and do not need to feed.
Unlike the trochophore, the veliger possesses many of the characteristic features of the adult. It possesses a muscular foot, eyes, tentacles, a fully developed mouth, and a spiral shell (in fact, the veliger of nudibranchs possesses a shell, even though the adult does not). Unlike the adult, however, the veliger possesses two ciliated semi-circular structures resembling fins or wings. These are collectively referred to as the velum, and are the larva's main means of propulsion, as well as straining food particles from the water.
The torsion of the visceral mass so distinctive of many gastropods occurs during the veliger stage. The sudden rotation of the bodily organs relative to the rest of the animal may take anything from three minutes to ten days, depending on species.
The veliger stage may last up to three months; it ends when the foot becomes sufficiently developed to allow locomotion, at which point the velum is lost, and the snail settles to the bottom to adopt its adult form.
Veliger of bivalves:
The veliger of bivalves like gastropods also typically follows a free-living trochophore stage. Shipworms, however, hatch directly as veligers, with the trochophore being an embryonic stage within the egg. Many freshwater species go further, with the veliger also remaining within the egg, and only hatching after metamorphosing into the adult form.
The shell of a bivalve veliger first appears as a single structure along the dorsal surface of the larva. This grows around the veliger's body, becoming folded into the two valves of the adult. The velum projects from between the valves, in front of the small foot.
As in the gastropods, the veligers of bivalves may either eat plankton, or survive off yolk retained from the egg.
The veliger attains the adult form when it sheds its velum. Some species spend a considerable time searching for an ideal habitat before metamorphosing, but others may settle on the nearest suitable substrate.
Veliger of scaphopods
The scaphopods, or tusk shells, have a veliger larva very similar to that of bivalves, despite the great difference in the appearance of the adults. The shell develops in a similar way, developing a bi-lobed form that surrounds the larval body. However, unlike bivalves, this never splits into two, and, in fact, fuses along the ventral margin, eventually becoming a tube that encloses the length of the body, and is open at both ends.
2.3 ECHINODERMATA
2.3.1 ORIGIN OF ECHINODERMATA
Echinoderms are one of the most beautiful and most familiar sea creatures. Forms such as the sea stars have become a symbol of sea life. Other forms such as brittle stars, sea urchins, sea cucumbers and sea lilies are also quite well known to the visitors on the sea shore. There are about 7000 species known in Echinodermata.
Echinodermata literally means “spiny or prickly skinned” (Gr., echinus – spiny; derma – skin) are refers to the conspicuous spines possessed by their test or skin. Jacob Klein (1734) first used this name for echinoids. The Greeks applied the name echinus to the hedgehog as well as the sea urchin, both having a prickly appearance.
The first universally accepted echinoderms appear in the Lower Cambrian period, asterozoans appeared in the Ordovician and the crinoids were a dominant group in the Palaeozoic. Echinoderms left behind an extensive fossil record. It is hypothesised that the ancestor of all echinoderms was a simple, motile, bilaterally symmetrical animal with a mouth, gut and anus. The larvae of all echinoderms are even now bilaterally symmetrical and all develop radial symmetry at metamorphosis. The starfish and crinoids still attach themselves to the sea bed while changing to their adult form.
The first echinoderms later gave rise to free moving groups. The evolution of endoskeletal plates with stereom structure and of external ciliary grooves for feeding was early echinoderm developments. The Palaeozoic echinoderms were globular, attached to the substrate and were oriented with their oral surfaces upwards. The fossil echinoderms had ambulacral grooves extending down the side of the body, fringed on either side by brachioles, structures very similar to the pinnules of modern crinoids. Their locomotor function came later, after the re-orientation of the mouth when the podia were in contact with the substrate for the first time.
2.3.1TYPES OF PEDICELLARIAE
The pedicellariae are the modified spines present in the space between the spines or in clumps around the bases of the spines throughout the body of echinoderms. They are microscopic in size and generally found in rare families of echinoderms. Commonly there are two types of pedicellariae.
1) Forceps or Straight type
It is a simple type of pedicellariae, in which the two jaws are straight in position and attached at basal end to the basal plate. When the pedicellariae are closed the jaws remain parallel and meet each other throughout the length like a forceps. There are two pairs of adductor and abductor muscles present between jaws and basal piece. The contraction and relaxation of these muscles open and closes the pedicillariae.
2) Scissor or Crossed type
These pedicellariae are relatively small in size and found near the white spines on the aboral surface. In this type, the basal piece is curved and cross each other at the anterior side. The inner lining of jaws is rough to form the denticles. As both jaws cross each other like a scissor, it is also called as scissor type of pedicillariae. The pair of adductor muscles and abductor muscles is attached with basal piece, which opens and closes the mouth of pedicellariae.
The pedicellariae perform different functions. They are useful for the protection of delicate skin, gills and keep the body surface free from foreign material. They are also called as offensive and defensive organs of star fish. They are also said to be sensitive in nature.
2.3.3 LARVAL FORMS IN ECHINODERMATA
Echinodermata shows complicated metamorphosis in the course of development. This may be may be direct or indirect. In direct development, the larval stages are missing, but in indirect development, there are various types of free swimming larvae. Echinoderms mostly have free pelagic larvae. Asterina passes a considerable time in the egg membrane and larvae are not pelagic. A few species keep the young in brood pouches until they attain adult hood. Larval forms of echinoderms are as follows.
1. Bipinnaria larva
i. Free swimming and bilaterally symmetrical.
ii. Angular in shape.
iii. The alimentary canal has mouth, esophagus, stomach, intestine and anus.
Ciliated bands are preoral loop and postoral loop.
iv. Preoral loop encircles mouth. Postoral loop surrounds the anus. Arms are bordered by the ciliary bands.
v. Arms are dorsomedian, ventromedian, preoral, anterodorsal, posterodorsaL pastoral and posterolateral.
Comment: This larval form is found in the Assteroidea and bears a close resemblance with the auricularia. In Luidia, it has a slender anterior part, which ends into the two wide arms.
2. Brachiolaria larva
1. It is found in Asteroidea
2. Bipininnaria changes into the brachiolaria larva.
3. It possesses three short additional arms, the brachiolar arms.
4. Arms are beset with warts to help in temporary adhesion and lack calcareous rods.
5. Arms have prolongation from the coelomic cavity.
6. Larva attaches to some object.
7. For some time, the anterior region acts as stalk, while the posterior part, having gut and coelomic chambers, converts into a young star, which detaches itself and lead a free life.
Comment: In Astropecten, the brachiolaria stage is omitted. The bipinnaria directly metamorphoses into the adult in 2—3 months.
3. Echinopluteus larva
1. It is microscopic. .
2. Free swimming and it develops within 30 days
3. Arms are 5 or 6 pairs, pigmented.
4. Calcareous skeleton supports the arms.
5. Posterolateral arms are simple, or thorny, or branched.
6. Mouth, oesophagus, stomach, intestine and anus are present.
Comment: It is found in Echinoidea. Locomotion of larva takes place by ciliated bands, which sometimes become thickened and known as the epaulettes. In Arbacia and Iclaris, the larvae develop the ciliated lobes, called the auricular lobes or auricles.
4. Ophiopluteus larva
1. This is free-swimming larva of Ophiuroidea.
2. Mouth, oesophagus, stomach, intestine and anus are present.
3. Arms are slender and are four pairs.
4. Calcareous skeleton supports the arms.
5. Posterolateral arms are the largest and directed forward giving the larva a V-shaped appearance.
6. Ciliated bands are present on the edges of the arms.
5. Auricularia larva
1. This is the larva of Holothuroidea.
2. It is bilaterally symmetrical, pelagic.
3. It measures 0.5 to 1 mm in length.
4. Single longitudinal ciliated band is present.
5. Lobes are well formed.
6. Calcareous rods are replaced by spheroids, or wheel like bodies.
7. Curved gut, with aciform stomach, hydrocoel, right and left somatocoels, is present.
6. Doliolaria larva
1. This type of larva is observed in Hothuroidea.
2. It has an apical sensory plate at anterior end with cilia and 4—5 ciliated bands on the body are seen.
3. Near the apical end, there lies an adhesive pit over the first ciliary band.
4. Stomodaeum or vestibule is found between the second and ciliated bands.
Comment: This larva is also called the pupa stage. In Cucumaria plancii, the auricularia stage is absent and embryo transforms directly into the doliolaria stage.
7. Antedon or yolk larva
1. It is the larva of Antedon. It is also known as doliolaria larva.
2. It is fee swimming and bilaterally symmetrical.
3. Ciliated bands are in form of 4 or 5 separate, transversely placed bands encircling the body.
4. Anterior ciliated ring is ventrally incomplete.
5. Larval mouth is ventrally placed between the second and third ciliated rings.
8. Cystidean or pentacrinoid larva
1. This larva is present in the crinoids.
2. It resembles closely with adult Pentacrinus.
3. The larva has an elongated narrow stalk and becomes broader.
4. Closed ectodermal vesicle is present.
5. The larva resembles closely the adult Pentacrinus.
Significance of echinoderm larvae
Except the larva of crinoids which becomes sedentary, the larvae of the Holothuroidea, Arteroidea, Echinoidea and Ophiuridea are fundamentally similar in features such as the peroral and anal loops, V-shaped ciliated bands, gut and enterocoelic coelom. They exhibit many common characters. Their respective groups might have been originated from a common ancestor and the hypothetical common ancestor is named the dipleurula. The dipleurula has oval body with flat ventral sides. It has a mouth and a digestive tube having the stomodaeum, stomach and protodaeum. The anus supposed to be formed by the atriopre.
Three pericoelomic vesicles and two water pores are present on the dorsal side. The dipleurula stage is bilaterally symmetrical. Bather advocated that dipleurula represents the common ancestor of the echinoderms. Bather fails to explain the water vascular system of echinoderm and its coelomic origin. Pentactula concept of echinoderm ancestry was adopted by Semon and later developed by Berry, Hyman and many others. According to them, pentactula larva occupies the next evolutionary rank over the dipleurula larva. The pentactula larva has five tentacles around the mouth and hydrocoel becomes separated from the rest of the coelom to form the water vascular system.
Recent inclination is to hold the view that the echinoderms have descended from a free-swimming common ancestor, possibly the pentactula larva. From the pantactula stage onwards, the divergence has started. Thus, studying larval forms of echinoderms would have significance in studying the phylogenetic consideration of the group.
Comment: Recent workers on other line have totally discarded the views of all early workers. They suggest that the larval affinities cannot act as a guide in establishing the phylogenetic relationships between different classes of the surviving echinoderms.
Relationship
1. Brachiolaria is modified form of the bipinnaria.
2. Bipinnaria bears a close resemblance with the auricularia of holothurians.
3. Pleutes can be regarded as the modified form of auricularia.
4. Ophiopluteus of ophiuridea are similar to echinoplutes of echinoidea.
Similarity between the ophiopluteus and echinopluteus must be due to convergent larval evolution. In asteroids, similar but somewhat more complex larva, bipinnaria or branchiolaria follows auricularia stage. Difference between Asteroid larvae may be due to divergent larval evolution.
Review questions
Long answer questions:
- What is metamorphosis? Describe the different types of metamorphosis in Insects.
- Give an account of different types of mouth parts occurs in insects.
- What is mimicry? Give the types of mimicry in insects.
- Give an account of larval forms in Crustacea.
- Give an account of larval forms in Mollusca.
- Give an account of larval forms in Echinodermata.
- Give the economic importance of Insects.
- Give the economic importance of Arthropoda.
- Give the economic importance of Mollusca.
- Give an account of different types of feet in mollusca.
- Give an account of different types of shells found in the mollusca with suitable example.
- Give the economic importance of Echinodermata.
- What is torsion? Explain how torsion occurs in Mollusca. Add a note its importance.
- What are pedicellariae? Give the different types of pedicellariae in Echinodermata. Add a note on their function.
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