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The following points highlight the fifteen interesting forms of Mollusca. The forms are: 1. Solen 2. Pecten 3. Mytilus 4. Teredo 5. Aplysia 6. Doris 7. Aeolis 8. Onchidium 9. Patella 10. Spiratella 11. Cypraea 12. Loligo 13. Octopus 14. Argonauta 15. Nautilus.
Molluscs: Form # 1. Solen:
Solen is a burrowing bivalve and is generally called ‘Razor shell’. The body is narrow and is covered by two thin, equal and straight shell valves (Fig. 16.48B). The umbo is shifted at the anterior end. One peculiar feature in this genus is the presence of a single-hinge tooth in each shell valve.
The mantle cavity is tubular and is formed by the fusion of the mantle lobes on the ventral side. The exhalant and inhalant siphons are small. There are a pair of elongated narrow gills in the mantle cavity. The foot is a highly muscular elongated organ which forms an efficient burrowing apparatus. The foot swells up very quickly by the inflow of blood during burrowing.
Molluscs: Form # 2. Pecten:
Pecten, commonly called ‘Scallop’, is a cosmopolitan marine bivalve (see Fig. 16.54). It feeds on micro-organisms caught during respiratory water current. The two shell valves are slightly unequal in size. The right valve is larger, convex in shape and the soft parts lie within this valve. The outer surface of the valves exhibits radiating striations and the concentric lines of growth take a wavy course.
The shell is beautifully sculptured. The hinge line between the shell valves is straight and is devoid of teeth. The margin of the mantle produces velar folds with numerous tentacles and stalked eyes at regular intervals. Only one large median adductor muscle is present. The foot is extremely reduced. It can swim by the activities of the shell valves.
Molluscs: Form # 3. Mytilus:
This sea-mussel is a sedentary bivalve (Fig. 16.55F) of cosmopolitan distribution. The valves of the shell are broad and rounded at the posterior side, and the anterior end is narrower. The umbo is shifted to the anterior side. Both the siphons are present but the inhalant siphon is less developed. The posterior margin of the mantle is fringed with tentacle-like processes. The foot is elongated.
The animal remains attached to the substratum by a bunch of byssus threads. The byssus threads project from the ventral side between the valves. It is a bunch of proteinaceous silky threads, produced by the byssus gland of the foot. The posterior adductor muscles are more developed than the anterior ones. The interlamellar and interfilamentar junctions are absent and are represented by a bunch of cilia.
Molluscs: Form # 4. Teredo:
This peculiar bivalve is well-known for causing tremendous damage to the ships and dock. It lives in holes made in the submerged portion of ship and wood. Because of its boring habit the body of Teredo has become extremely modified. It has an elongated vermiform body and is popularly called as the ‘Shipworm’ (Fig. 16.48C).
The shell valves are very small and are situated in the anterior end of the body. The shell has a rough surface which acts as the cutting and boring machine. It has a very minute foot reduced into a sucker to adhere to the wall of the hole. The sucker projects through a small aperture left by the mantle lobes and the rest of the mantle lobes are united. The mouth is situated between the shell valves.
The body proper and the inhalant and exhalant siphons are extremely elongated. The siphons are united at the anterior side, and the posterior portions are free. Just at the region of separation of the siphons there lie a pair of calcareous pallets which close the opening of the hole like a lid when the body and the siphons are withdrawn.
Teredo feeds on sawdust and small plankton brought by respiratory water current. It has the remarkable property of digesting the cellulose.
Molluscs: Form # 5. Aplysia:
Aplasia is a very peculiar gastropod and has almost a cosmopolitan distribution. It is recorded that it attains a size of about 1 m in length. Because of the superficial resemblance with hare, particularly in the head region, it is commonly called the ‘Sea-hare’ (Fig. 16.49A). The head bears two pairs of tentacles.
The posterior pair is comparatively smaller and is called rhinophores. These are olfactory in function. The anterior pair of tentacles are longer in size and are called cephalic tentacles. The foot is broad and bears lateral projections, called parapodia, as the organs of swimming. The foot gives origin to a very short, distinct tail at the posterior end.
The posterior part of the foot is usually adhesive. The mantle is extended upwards as a thin membrane and the shell is concealed by the mantle outgrowth. The shell is reduced to a small flattened piece which is internally placed. The mantle cavity opens to the right side of the body through a longitudinal slit.
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The ctenidium is visible along the opening of the mantle cavity. The floor of the mantle is provided with large subepithelial gland cells, called opaline glands. Aplysia is notable for discharging purple or milky fluid to the surrounding water to conceal itself from enemies.
This fluid is secreted by another type of gland in the inner mantle epithelium, called Blochmann’s glands. The nervous system is of ethyneurous type, i.e., attainment of secondary symmetry after double torsion. They are of bisexual forms producing both sperms and eggs. The gonoduct is single and opens to the exterior through a common genital opening.
Molluscs: Form # 6. Doris:
Doris, popularly called ‘sea-lemon’, is a curious gastropod (Fig. 16.49B), where the shell and ctenidium are totally absent. The anus is located posteriorly and is mid-dorsal in position. It is surrounded by a circlet of anal gills which act as respiratory organs. The body of Doris comprises in a more or less ovoid mass with a convex warty dorsal side.
The head bears a pair of rhinophores beset with calcareous spicules. The foot has a flat creeping sole. The digestive gland is an un-branched compact mass. The nervous system is of ethyneurous type. The sexes are united and the gonopore is asymmetrically placed on the right side of the body.
Molluscs: Form # 7. Aeolis:
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Like Doris, Aeolis is also a nudibranch and commonly called sea slug. It has an elongated shell-less body. It has two pairs of tentacles, the anterior pair are called cephalic tentacles and the posterior pair are designated as rhinophores (Fig. 16.49C).
The rhinophores are simple. The radular tooth is arch-like and is devoid of median notch. True gills are absent and respiration is done through transverse rows of brightly coloured secondary cylindrical cerata which also contain extensions of the digestive system. The cerata open to the exterior through openings.
Aeolis feeds on sea-anemones. The cnidoblasts of the anemones taken inside the body of Aeolis usually escape digestion and migrate to the tips of the cerata. These are called the cnidosacs which probably act as defensive organs. It is claimed by many that on proper irritation the cnidosacs discharge their contents towards the prey or enemies.
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But such a phenomenon is not witnessed by any worker. However, existence of cnidosacs in the body of Aeolis is a unique instance and the significance of cnidosacs in the body of Aeolis is not fully understood.
Molluscs: Form # 8. Onchidium:
This curious slug-like gastropod is placed under pulmonata. As regards habits and general appearance, Onchidium resembles superficially the chitons. Like other gastropods two pairs of tentacles are present. It has an ovoid naked body. The foot has a very broad flat creeping sole.
The mantle covers the dorsal side of the body and is beset with numerous warty tubercles. These warty bodies may bear eyes and may also transform into bunch of branchial filaments as respiratory organs. The true ctenidium and mantle are lacking. The head possesses two retractile tentacles, each ending in an eye (Fig. 16.50A).
A pulmonary chamber is present in the posterior part of the body. The sac opens to the exterior through a small aperture (pneumostome) near the anal opening. Onchidium is hermaphrodite. The female gonopore is situated posteriorly near the anal opening, while the male genital aperture is located anteriorly near the base of the right retractile tentacle.
Molluscs: Form # 9. Patella:
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Patella is a small, sluggish gastropod inhabiting the rocky beaches. Patella lives upon marine vegetation. The dorsal side of the body is enclosed by a roundish shell. The shell is raised into a conical elevation. The operculum is absent. The ventral side of the body is occupied by a broad creeping foot (Fig. 16.50B).
The foot is also employed as an adhering organ. The mantle is highly pigmented. The head bears a pair of large sensory tentacles and a pair of eyes. Between the foot and mantle, there exists a series of secondary branchiae.
The primary gills are obliterated excepting a pair of vestiges. A pair of osphradia and a pair of oesophageal pouches are present. It has a single auricle. There are two kidneys, of which left one is smaller in size. Sex reversal is very common in Patella. Fertilization is external.
Molluscs: Form # 10. Spiratella (Limacina):
Spiratella is a typical pteropod. Like all other forms, Spiratella leads pelagic life and swims by flapping the parapodia. The foot develops anteriorly into highly muscular wing-like projections, called parapodia (Fig. 16.51A). The posterior part of the foot is greatly reduced.
It has a spirally coiled calcareous shell, and an operculum is present. The shell assumes various shapes in other related genera. Spiratella has a dorsal mantle cavity but without ctenidium. They usually swim in large numbers and are generally called the ‘butterflies of the sea’.
Molluscs: Form # 11. Cypraea:
This type of gastropod is a very familiar form of mollusc. The shell of this form is used as curio, ornament and decorative article. It was used as currency in ancient days. It is a very beautifully coloured form living on rocky background, specially in the coral reefs. The shell has a large oval single whorl.
The dorsal side is convex and the ventral side is flat with a narrow longitudinal opening extending along the length of the shell (Fig. 16.51B). This opening is toothed on both sides. The lateral sides of the mantle folds are reflected over the dorsal side of the shell to make it internal. The shell lacks periostracum and the surface is smooth.
Molluscs: Form # 12. Loligo:
Loligo is avery common cephalopod and is commonly called squid. It has almost the same anatomical organisation as that of Sepia, but it possesses certain individual peculiarities.
Loligo has an elongated spindle- shaped or torpedo-shaped body (Fig. 16.52A). The body is divided into head, tapering trunk and a narrow neck between them. The head bears ten arms of which eight are short and two are usually elongated like those of Sepia.
The fins are conical and are confined to the posterior end of the body. Figure 16.52B shows the internal organisation of Loligo. The shell is internally placed and resembles old styled quill-pen or called gladius (Fig. 16.52C). The caecum is very large in size and bears an anterior curved diverticulum.
The liver is a single lobed structure. An ink-sac is present. The heart consists of two auricles and a single ventricle. An additional rounded branchial heart is present at the base of each gill. The excretory system comprises in two triangular whitish bodies, called kidneys. The nervous system is highly developed and resembles closely the nervous system of Sepia.
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The eyeball of the Giant Squid is the largest of all animals.
Molluscs: Form # 13. Octopus:
Octopus, popularly called devil fish, is a very well-known molluscan form. It has a cosmopolitan distribution and lives at the bottom of the sea. It is nocturnal and lives on crabs, fishes and other molluscs. It has a roundish body with a head. The shell is absent Eight elongated arms of equal sizes are present (Fig. 16.55B). Each arm carries two longitudinal rows of sessile suckers.
In male Octopus the third right arm is modified into a spoon-shaped hectocotylized arm. The animal can crawl by its arms and can also swim backward by ejecting jet of water from the funnel. Octopus has the ability of ejecting inky fluid to the surrounding water as a defensive device. Octopus has the property of changing the colour of the body when it is irritated. The nervous system is highly developed.
Molluscs: Form # 14. Argonauta:
Argonauta, popularly known as ‘Paper Nautilus’, is closely related to Octopus. Sexual dimorphism is distinct in Argonauta. The male is much smaller than the female and lacks shell. But the female is very large in size and possesses a thin fragile external shell (Fig. 16.53). The shell is symmetrical, simply coiled and lacks internal septa.
The shell is mainly used to carry eggs. Of the eight arms, two dorsal arms become specialised and are applied to the outer surface of the shell. The terminal end of these two arms become expanded to form disc-like structure. The shell in females Argonauta is secreted by these two arms and is not produced by the mantle as the case in other molluscan forms. Like that of Octopus, the third right arm in males is hectocotylized.
Molluscs: Form # 15. Nautilus:
Nautilus is gregarious, deep-sea inhabitant, nocturnal cephalopod found in Indian and Pacific Oceans. It is popularly called the ‘Pearly Nautilus’. The external shell is flat and coiled spirally in one plane. It is divided internally into many compartments by internal septa (Fig. 16.53B). The chambers increase in size from inner to the outer side of the spiral, i.e., the outermost chamber is the largest.
The body proper is lodged in the largest chamber and the other chambers remain either empty or filled up with gas which helps in floatation. The internal septa bear perforation in the middle.
A narrow, tubular, vascular prolongation of visceral region with a thin calcareous enclosure is situated in the inner-side of the shell, called siphuncle or siphonal tube which passes through the middle of each septum and removes fluid from the former chambers and regulates the buoyancy of the animal by removing fluid and gas to the outmost chambers.
The mouth is surrounded by radially arranged prehensile tentacles. The number of tentacles varies from 60 to 90. The tentacles are devoid of suckers. On the dorsal side, two tentacles become thicker to form a protective hood above the mouth. The outer surface of the shell has alternate brown and dark bands. The inner surface is pearly white, The ink-sac is absent.
