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Phylum Mollusca

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Phylum Mollusca Animal Diversity: (Non-Chordates) Phylum : Mollusca Ranjana Saxena Associate Professor, Department of Zoology, Dyal Singh College, University of Delhi Delhi e-mail: [email protected] 24th September 2007 CONTENT 1. GENERAL CHARACTERISTICS 2. PILA GLOBOSA a) Habit and Habitat b) Morphology c) Coelom d) Locomotion e) Digestive System f) Respiratory system g) Circulatory System h) Excretory System i) Nervous System j) Sense organs k) Reproductive System 3. SEPIA a) Habit and Habitat b) Morphology c) Shell d) Coelom e) Locomotion f) Digestive System g) Respiratory System h) Circulatory System i) Excretory System j) Nervous System k) Sense Organs l) Reproductive System 4. ANCESTRAL MOLLUSK 5. SHELL IN MOLLUSCA 6. FOOT AND ITS MODIFICATION 2 7. GILLS AND ITS MODIFICATION 8. MANTLE 9. TORSION IN MOLLUSCA 10. PEARL FORMATION 11. CLASSIFICATION 12. BIBLIOGRAPHY 13. SUGGESTED READING 3 PHYLUM MOLLUSCA The word Mollusca is derived from the latin word mollis which means soft bodied. GENERAL CHARACTERISTICS • It is the second largest phylum of invertebrates consisting of more than 80,000 living species and about 35,000 fossil species. • The adults are triploblastic, bilaterally symmetrical animals with a soft unsegmented body. However, the bilateral symmetry may be lost in some adult mollusc. • Majority of them are enclosed in a calcareous shell. The shell may be external or in a few molluscs it may be internal, reduced or absent. • They have a well marked cephalisation. • The body is divisible into head, mantle, foot and visceral mass. • The visceral mass is enclosed in a thick muscular fold of the body wall called mantle which secretes the shell. • Body cavity is a haemocoel. True coelom is schizocoelic and reduced to cavities of pericardium, gonads and kidneys. • The buccal cavity contains a rasping organ called radula. It helps the animal in feeding. In some of the molluscs, however, it is secondarily lost. • A ventral muscular foot is an adaptation for locomotion, primarily for crawling. In some, it is secondarily modified for swimming or burrowing. • Paired gills or ctenidia are the primary aquatic respiratory organs. Some of the molluscs may possess pulmonary sac for breathing atmospheric oxygen. • Blood vascular system is of the open type with dorsal heart and a few blood vessels. However cephalopods show a tendency towards a closed system. • Respiratory pigment is haemocyanin. • Excretion is by a pair of kidneys or nephridia or organ of Bojanus. • An olfactory organ, the osphradia is situated at the base of the gills. The osphradia tests the purity of water that enters the mantle cavity. • Majority of them are dioceous but a few of them may be hermaphrodite. • Development is generally indirect having larval stages but direct development is also found in some molluscs. Trocophore larva, veliger larva and glochidium larva are the characteristic larvae of mollusc that have an indirect development. Molluscs may be terrestrial or aquatic (freshwater or marine). They may be found in all habitats, deserts, forests, lakes, rivers, abysses of sea, coral reefs, underground or even as parasite in the body of other animals. They may be found clinging to the rocks, crawling, swimming, burrowing or even digging. They have a diversified feeding habit, may be herbivores, carnivores or omnivores. They vary in size from giant squids (nearly 2m long) to little snails, a millimeter long. Molluscan shell have always been economically important, having served as money in early days. They have been used in jewellery and buttons. Scallops, oysters, squids and octopus are important food items. Since phylum mollusca has wide range of structural diversities two animals namely pila and sepia will be described in details for a better understanding of the phylum. 4 PILA GLOBOSA Commonly known as apple snail or pond snail. HABIT AND HABITAT: Pila globosa is amongst the largest fresh water molluscs that have invaded various kinds of habitats. They live in fresh water pools, ponds, lakes and paddy fields where succulent water plants are in abundant. Sometimes they may be found in running streams also. They are herbivores and aquatic plants like Vallisnaria and Pistia constitutes its food. Pila is a voracious feeder and feeds on plant scrapings which are cut with the help of chitinous jaws and radular teeth. The snail creeps slowly with the help of ventral muscular foot. While feeding or moving, the animal does not leave the shell completely but carries it on its back. Whenever it senses danger, it retreats into its shell and closes the shell aperture with the operculum. When the snail is in a quiescent state, it retracts its body into the shell. But when active, the head and foot protrudes out of the shell and the animal creeps slowly. During unfavourable conditions, when the pond dries up as is seen in summers, it completely retracts their body within the shell and remain torpid. This period of inactivity is called aestivation or summer sleep. They can live for years in this condition. When the conditions are favourable, like in rainy season they once again become active. They are amphibious i.e. can respire both in water and on land. In water they respire with the help of ctenidia while on land they respire by pulmonary sac. MORPHOLOGY: The soft body of the animal is enclosed in a thick, calcareous shell composed of a single piece and hence the shell is called a univalve shell. The shell has a conical structure, spirally coiled around a central axis called the columella. The columella is hollow and its cavity opens to the outside by umbilicus. A shell with an umbilicus is called as umbilicate or perforate shell. Each revolution around the axis is called a whorl. The small rounded tip of the shell is called the apex and the whorl surrounding it is called the apical whorl (Fig. 1). The apex of the shell is the smallest and is regarded as the oldest part of the shell. The lower most whorl is the largest and is known as the body whorl. The whorl above the body whorl is called the penultimate whorl. All the whorls except the body whorl together are called spire (Fig. 2). Internally, all the whorls freely communicate with one another and there is no separating partition between them and thus the shell is known as unilocular. Externally, there is a line at the junction of the two successive whorls which is known as suture. The penultimate whorl and the body whorl are large enough to enclose most of the body parts. The outer surface of the shell is marked with numerous fine vertical lines called lines of growth. In addition to these lines of growth, there are few prominent deep vertical lines along the entire length of the shell. These are called varices (singular varix) and indicate fresh growth after every aestivation or hibernation. The body whorl opens to the outside by a wide aperture or mouth, the outer margin of which is called the outer lip while the inner one is called the columellar lip. The margin of the aperture is smooth and rolled out and is known as peristome. The aperture can be closed by a flat calcareous plate called the operculum (Fig. 1), which is attached to the dorsal side of the hind part of the foot. The operculum is secreted by the glandular cells in the foot. A distinct elliptical area is seen on the inner surface of the operculum and is called the boss. Opercular muscles are attached to the boss. The operculum shows many concentric rings of growth arranged around a small central axis called the nucleus. When the foot is withdrawn into the shell, the operculum fits into the mouth of the shell and closes it. 5 The whole body is enclosed within the whorls of the shell and is attached to the columella by columellar muscles. The columellar muscles arise from the foot and prevent the animal from extending out of the shell beyond a certain limit and also helps to withdraw the body into the shell. Beneath the shell is the soft body made up of head, foot, visceral mass and mantle or pallium (Fig.3). Head and foot together form the head-foot complex which retains the bilateral symmetry. The visceral mass and the pallium together form the visceropallium which has secondarily lost the bilateral symmetry. HEAD: The head is the muscular part of the body which hangs over the foot and projects out as a retractile snout. Head bears two pairs of contractile tentacles arising from behind the snout from the dorsal surface of the head.. The anterior pair of tentacles are called the cephalic tentacles or labial palps and are prolongations of the sides of the head. The posterior pair of tentacles are long, fleshy and contractile and are the true tentacles. The tentacles are hollow and are capable of extension and retraction. Eye is present on a small club shaped stalk called ommatophore just behind the true tentacles (Fig. 3 and 4). Mouth is a vertical slit like aperture lying ventrally between the bases of the cephalic tentacles. FOOT: A large more or less conical foot adapted for creeping movement is present below and behind the head. The anterior part of the foot is round and the posterior dorsal surface bears the operculum (Fig. 3). The foot is highly muscular and contains pedal glands, the secretions of which form a slime trail during locomotion. The head and foot together constitute the head-foot complex. The two are connected to the visceral mass through a slight constriction. 6 penultimate whorl apex of shell sutures spire body- whorl peristome operculum lines of growth nucleus umbilicus outer lip inner or columellar lip Fig.
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