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The following points highlight the twenty representative types of Molluscs. The types are: 1. Chiton 2. Neopilina Galatheae 3. Haliotis 4. Fissurella 5. Patella 6. Aplysia 7. Doris 8. Eolis 9. Limax 10. Dentalium 11. Mytilus 12. Pecten 13.Ostrea 14. Teredo 15. Solen 16. Sepia 17. Loligo 18. Octopus 19. Argonauta 20. Nautilus.
Molluscs: Type # 1. Chiton:
Chiton (Fig. 62.1) is a sluggish, marine animal found attached to the rocks, empty shells and corals between tide marks. Body is elongated, bilaterally symmetrical and dorsoventrally compressed and consists of shell, foot, mantle and the visceral mass. Shell is calcareous present on the dorsal side and composed of eight overlapping plates. Head is not distinct. Eyes and tentacles are absent.
Foot is ventral, muscular with a flat sole extending along the whole length of the body. It serves for creeping and adhering to the substratum. Mantle covers greater part of the body and partly covers the edges of the shell plates. Mouth and anus are at opposite ends.
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Numerous pairs of bipectinate ctenidia lying on either side of the body in the mantle groove. Sexes are separate gonad is single and median and gonoducts are paired. Development includes a trochophore larva.
Molluscs: Type # 2. Neopilina Galatheae:
Neopilina galatheae (Fig. 62.2) is one of very few living members of the Monoplacophora. Neopilina is a deep sea mollusc of special interest because it illustrates the primitive occurrence of metamerism in molluscs.
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Neopilina was discovered in 1952. In Neopilina the single symmetrical shell is low, somewhat patelliform and exogastrically coiled.
The largest specimen known is 37 mm long, 35 mm wide and 13 mm high. Neopilina appears somewhat chiton-like from the ventral surface. The central circular foot is separated from the encircling mantle by a pallial groove Which contains five pairs of lamellated gills with a ciliated epithelium.
The head is inconspicuous with two small pre-oral tentacles and two oral tentacle tufts around the mouth. Anterior to the mouth is a velar ridge with two flaps. Unpaired anterior and posterior lips border the mouth.
A feeding furrow extends between velar ridge and tentacle tufts. At the posterior end is the anus. Pharynx contains a radula. The stomach contains a crystalline style. The muscular system is complex. Six pairs of nephridia are located in the pallial fold. Sexes are separate. The food of Neopilina consists of radiolarians and other materials of the bottom.
Molluscs: Type # 3. Haliotis:
Haliotis (Fig. 62.3) is marine and found attached to the rocks between tide marks.
Haliotis is commonly called ear-shell. Shell is ear-shaped with small flattened spire, very large aperture and perforated by a series of marginal slits through which projects the tentacular process of the mantle. Operculum is absent. Eyes are borne upon stalks at the outer bases of tentacles. Foot very large with epipodia projecting through the shell.
Mantle cavity is spacious and contains two bipectinate ctenidia or gills, the right being smaller. Two auricles and two nephridia are present. Haliotis or Abalone is of great economic importance. Shell is used for the preparation of beautiful buttons, buckles, etc., and flesh is eaten in Japan, China and in many cities along the Pacific coast.
Molluscs: Type # 4. Fissurella:
Fissurella (Fig.62.4) is commonly known as keyhole limpet. It resembles closely to the true limpets. Shell is oval, conical and depressed without an operculum. The mantle and the apex of the shell have an apical lobe leading into the branchial chamber. The foot is large and ventral; it is used as a sucker for attaching to the rocks. Foot bears on either side an epipodial ridge bearing a row of cirri.
Molluscs: Type # 5. Patella:
Patella (Fig. 62.5) is a sluggish, marine gastropod, found attached to the rocks and feeding on minute algae. Patella a limpet, is a small oval gastropod. Shell is oval and rounded without operculum. Head is distinct; bears a pair of stout, sensory tentacles and eyes.
Foot is ventral, broad and flat; used for creeping and adhering. True mantle cavity is restricted anteriorly and the ctenidia or gills have disappeared.
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Secondary mantle cavity extends all round between the foot and mantle and contains a series of pallial gills or secondary branchiae for respiration. Radula composed of very few, strong hooked teeth in each row. Heart with single auricle. Patella is eaten by poorer class in several countries like France, Italy and Ireland.
Molluscs: Type # 6. Aplysia:
Aplysia. (Fig. 62.6) is found crawling in sea weeds in most parts of the world from the Arctic to the Antarctic. Aplysia is commonly known as sea hare. The body is soft and lumpy with a thin flexible plate-like shell almost completely covered by mantle. Head bears two pairs of tentacles, the anterior being larger and ear-like, while the posterior pair is olfactory, each bearing an eye at its base.
Mantle cavity opens on the right side with the ctenidium pointing backwards. Visceral mass is raised into a prominent hump. Foot is broad and ventral and bears a pair of lateral fleshy outgrowth, the parapodia which help in swimming. Anus lies at the posterior end. Hermaphrodite with a single generative duct. In the wall of the mantle is a gland which secretes a purple pigment. Aplysia feeds mainly on the sea weeds.
Molluscs: Type # 7. Doris:
Doris (Fig. 62.7) is a sluggish marine animal moving between weeds and feeding on the encrusting organisms. Doris is commonly known as the sea lemon. The body is bilaterally symmetrical, short, flat and oval with convex dorsal surface. Skin is tough and contains calcareous spicules of various types arranged symmetrically.
Head bears a pair of short retractile tentacles or rhinophores. Foot is ventral with a broad sole for creeping. Dorso-median anus lies posteriorly and is surrounded by secondary plumose retractile branchiae. Hermaphrodite. Doris exhibits complete detorsion as shown by the absence of shell mantle and ctenidium.
Molluscs: Type # 8. Eolis:
Eolis (Fig. 62.8) inhabits shallow waters, found often crawling on the underside of sea weeds. Eolis is commonly known as sea slug. Body is ovate, soft about 1-2 cm in length and orange with brown spots in colour. Head bears two pairs of cylindrical retractile tentacles and sessile eyes at the base of the posterior pair of tentacles.
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Foot is muscular and ventral and serves for locomotion. Cerata or secondary branchiae are numerous, cylindrical processes distributed over the dorsal surface. Shell, mantle and true gills are absent. Hermaphroditic, common gonad is protrandric. The animal is said to feed on hydroids which are digested and their un-discharged nematocysts are collected in the cerata and discharged on irritation.
Molluscs: Type # 9. Limax:
Limax (Fig. 62.9) is a terrestrial cosmopolitan in distribution and found in gardens over damp soil and cultivated lands. Limax is commonly known as gray slug. Body is elongated and tapering behind and is divisible into head, foot and visceral hump. Head bears two pairs of retractile tentacles, the posterior part of tentacles bears small black eyes at their lips.
Mantle forms a shield-like area over the anterior portion of the body. Shell is internal, thin and rudimentalry lies embedded in mantle. Foot is well developed, blunt anteriorly and tapering posteriorly. Pulmonary aperture lies on the right side of the mantle. Hermaphrodite. Nocturnal and herbivorous.
Molluscs: Type # 10. Dentalium:
Dentalium (Fig. 62.10) is marine and found in the sand at great depth. It is commonly known as tusk shell.
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Body is bilaterally symmetrical and enclosed in a tubular shell open at both ends. Mantle folds are fused ventrally to form a tube enclosing the body. Head is vestigial, bearing the mouth which is surrounded by a circlet of retractile tentacles, the captacula with sucker-like ends. Foot is long and conical, protrudes through the anterior opening of the shell and is used in burrowing.
Well developed radula is present. Anus lies behind the base of foot. Gills are absent.
Respiration by transverse folds in the lining of mantle. A pair of nephridia is present with their external openings on either side of the anus. Vascular system is poorly developed without distinct head. Nervous system simple and consists of cerebral, pleural and pedal ganglia. Eyes absent. Otocysts present. Sexes separate. Development includes veliger larva.
Molluscs: Type # 11. Mytilus:
Mytilus (Fig. 62.11) is marine, sedentary, cosmopolitan and found attached to the rocks between tide marks. Mytilus is commonly called sea mussel. Body is enclosed in a wedge- shaped shell of two equal valves, which is pointed in front and rounded behind. Byssus threads protrude from between two shell valves ventrally by which it is attached to stones and rocks.
Mantle is bilobed and forms an exhalant siphon posteriorly. Foot is cylindrical, elongated with a ventral groove continuous with a byssus pit. Anterior adductor muscle is weaker, while posterior adductor muscle is strongly developed. A pair of gills is present, each gill is provided with filaments. A pair of simple eyes is present. Sexes are separate. Gonads extend into the mantle.
Molluscs: Type # 12. Pecten:
Pecten (Figs. 62.12 and 62.13) is marine, free swimming and cosmopolitan in distribution. Pecten is commonly known as scallops. Shell is beautifully ribbed by radiating lines. The two shell valves are unequal, the right being larger and more convex and the animal rests on this valve.
Single large adductor muscle is divided into two parts and the larger of these serves for swimming movements. Foot is very much reduced. Two large and crescentic gills are present. Large number of stalked eyes are present at regular intervals along the edges of the mantle. Hermaphrodite. Ovary is pink and testis is cream coloured.
Molluscs: Type # 13. Ostrea:
Ostrea is marine, sedentary animal found attached to rocks and stones, etc. Ostrea is commonly known as edible osyter.
Shell surface is coarse, irregular and ruffled. Two valves of the shell are unequal,the left being larger and attached to the substratum. Single adductor muscle divided into two parts. Foot is absent. Heart with two fused auricles. Individuals function alternately as males and females. Ostrea is used as favourite food in many countries.
Molluscs: Type # 14. Teredo:
Teredo (Fig. 62.14) is commonly known as ship worm. It is marine, found in burrows made in submerged wood of ships and boats. Body is elongated and vermiform. Mantle is tube-like and opens anteriorly. Foot is very much reduced. Shell is reduced and consists of two small valves enclosing only the anterior region of the body, provided with a rough and ridged surface.
Siphons are long, the two siphons are united most of their lengths and provided with calcareous pellets for protection. Mantle cavity is long and contains a pair of elongated gills. Teredo bores into submerged wood reducing it into saw-dust by the rotatory action of the two shell valves. Saw-dust is swallowed by the animal and digested by the enzymes secreted by the digestive gland.
Molluscs: Type # 15. Solen:
Solen (Fig. 62.15) is commonly known as razor clam or razor shell. The shell is long and cylindrical, shell valves are narrow and straight, they gape at both ends, the um-bones are flat and terminal at the anterior end, hinge ligament is long and external, and there is one hinge tooth in each shell valve. The foot is long and cylindrical.
The mantle forms short exhalant and inhalant siphons posteriorly, the two mantle lobes are also fused ventrally leaving only an anterior pedal aperture through which the foot protrudes. Mantle cavity has long, narrow ctenidia in which the surface area of lamellae has been increased by folding.
Blood corpuscles have hemoglobin, unlike most Mollusca in which blood has haemocyanin. Siphons have pigmented refractive cells sensitive to light. Solen burrows actively and rapidly deep into the sea mud, but it jerks itself forward by withdrawing the foot and ejecting water through the siphons. In certain areas clams are used as human food.
A similar razor clam Ensis is common in the Indian Ocean, its shell is about 20 cm long with a slight bend which makes the dorsal side concave and ventral side a little convex, the right shell valve has two hinge teeth and left one has three; siphons are very short.
Molluscs: Type # 16. Sepia:
Sepia (Fig. 62.16) is commonly known as cuttlefish. It is a marine form, found in the shallow waters and world-wide in distribution. Body is bilaterally symmetrical, dorsoventrally flattened and is divisible into head, neck and trunk. Head bears a pair of large eyes and five pairs of arms surrounding the mouth.
Of the five pairs of arms, four pairs are short and stout bearing four longitudinal rows of suckers on the inner flat surface. The fifth pair of arms is known as tentacles which are comparatively longer and narrower and provided with suckers only towards their free ends. Neck is constricted and connects the head with the trunk. Trunk is elongated and shield-shaped bordered by narrow lateral fold on either side.
Mantle is thick and muscular enclosing a large mantle cavity on the ventral side which contains the viscera. Funnel is tubular opening into the mantle cavity. Shell is internal and enclosed in a shell sac in the mantle on the upper surface. Chromatophores are present in the deeper layers of integument over the entire surface.
A pair of large plume-shaped ctenidia or gills, one each side of the mantle cavity performs the respiratory function. Single kidney is excretory organ. A pear-shaped ink sac lies over the posterior ventral surface. Sexes are separate. In male the left fourth arm is hectocotylised.
Molluscs: Type # 17. Loligo:
Loligo (Fig. 62.17) is commonly known as squid. Body is spindle or torpedo- shaped and is divisible into head, foot and visceral hump. Head is short and bears a pair of large eyes and a central mouth surrounded by ten arms. Foot is modified into the funnel and the ten arms. Eight arms are short, stumpy and non-retractile, while two are long, slender and retractile tentacles used for capturing the prey.
Inner surface of both arms and tentacles is provided with suckers. Funnel is muscular tube extending out beyond the edge of the mantle collar beneath the head. Visceral hump is long and pointed, bears two dorsolateral triangular fins. Mantle is thick and muscular encloses the visceral mass and mantle cavity.
Shell is internal, feather-shaped plate concealed beneath the skin on the anterior surface. Two elongated gills are present in the mantle cavity. Two nephridia or kidneys are also present. An ink sac is present and serves for defence. Sexes are separate. Loligo is used as food by Chinese and Italians and also as bait for marine fishing. Loligo is a marine form and has a worldwide distribution.
Molluscs: Type # 18. Octopus:
Octopus (Fig. 62.18) is popularly known as devil fish. Body is globose with large head and trunk region.
Head bears a pair of eyes and eight elongated equal arms webbed at the base which surrounded the mouth. Each arm bears suckers arranged in two rows. Third right arm in male is modified into a spoon- shaped structure which serves for transferring the spermatophores into the mantle cavity of the female for fertilizing the ova. Shell is absent.
Mantle encloses the mantle cavity and the visceral mass. Octopus produces inky-fluid which diffuses in water and forms a smoky screen for defence from the enemies. Octopus is marine, cosmopolitan in distribution and found on the bottom of the sea.
Molluscs: Type # 19. Argonauta:
Argonauta (Fig. 62.19) is commonly known as Paper Nautilus. Sexual dimorphism is remarkable and well marked. Male is small about 2.5 cm in length and without shell. Female is about 20 cm in length and possesses a thin, transparent unilocular shell which is secreted by extremities of two dorsal arms.
Shell in female is used for protection of eggs. Mouth is surrounded by four pairs of elongated arms provided with suckers.
Third arm in the male is hectocotylised and is used for conveying the spermatophores in the mantle cavity of female. Funnel and eyes are well developed. Two plume-like ctenidia and two kidneys are present. Argonauta is found in all warmer seas. Female lives in deep waters and comes up only at the spawning season.
Molluscs: Type # 20. Nautilus:
Nautilus (Fig. 62.20) is commonly called as Pearly Nautilus. Body is enclosed in a calcareous spirally coiled many chambered shell. The animal occupies the outermost and largest chamber into which body can be withdrawn for protection. The chambers are separated by a system of septa which are perforated in the middle and traversed by narrow tubular vascular prolongations of the visceral mass, the siphuncles.
Except the outermost chamber, all the chambers are filled with air so that the shell can float and animal can swim easily.
Body of the animal consists of head bearing eyes and a system of tentacles and a sac-like trunk. Mouth is situated at the end of the head and surrounded by numerous lobes bearing two rows of retractile tentacles. Outer rows of tentacles are borne on a muscular ridge forming the hood which serves as an operculum.
Arms and suckers are absent. Two pairs of gills, two pairs of kidneys and two pairs of auricles are present. Sexes are separate. It is nocturnal feeding on crabs and shell fishes. Nautilus is used extensively for ornamental purposes. Nautilus is marine, found living in shallow waters near the shores and coral reefs in the Indian and South Pacific Oceans.
