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Phylum : Mollusca
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- 2. General Characteristics 50,000or more species of molluscs. They share 3 major sets of characteristics: Body enclosed by a mantle that secretes a shell made of calcium or some other stiff structure. A mantle cavity between the mantle and the internal organs; the anus, reproductive, and excretory ducts open into the mantle cavity. A ventral, muscular foot that is highly modified among the various groups of molluscs. 2
- 3. General Characteristics Cont’d Free-living, multicellular animals Invertebrates (lack a backbone) Unsegmented Have a true coelom Bilateral symmetry Simple to complex nervous system 3
- 4. Habitats & Niches Some mollusc habitats include: Marine (mostly) Freshwater (a few) Land (a few) Mollusc niches (life-style): Most live on the ocean bottom and are mostly sedentary Some are free-swimming (the cephalopods) 4
- 5. The six majormollusc classes Those 6 classes are: Monoplacophora (mono=single; placo=shell; phora=bearing) Ex. Neopilina Aplacophora (a=without) Ex. Neomenia Polyplacophora (poly=many) Ex. Chitons Gastropoda (gastro=body cavity; poda=foot) Ex. Common garden snail, abalone, turban snail Scaphopoda Ex. Dentalium Cephalopoda (cephalo=dealing with head; poda=foot) Ex. Squid, octopus, etc. 5
- 6. Shell in Mollusca Thesoft, unsegmented bodies of mollusk is covered by hard and calcareous shell which acts as protective covering. Shell may be external or internal Study of mollucscan shell called conchology. 6
- 7. CLASS: Scaphopoda (Dentalium) Shell opens on both ends External ,cyndriccally tubular, curved and tapered. Oral end is wider, shape like elephants tusk, white in colour with grooves on entire shell. In Cadulus is cucumber shaped. 7
- 8. Class Aplacophora Shellis absent, mantle is thick with calcareous spicules. Acts as protective covering in Solenogaster. 8
- 9. Class Monoplacophora NeopilinaSingle shell dorsal large cup like shell is present. shell is thin, subcircular with raised and curved apex. 9
- 10. Class Polyplacophora Chitonsare the only genus in the class Polyplacophora. Their specific characteristics: Posses a shell that consists of 8 overlapping plates. Overlapping calcareous plates forms a solid armour covering the dorsal surface. The cephalic and anal shell plates are keeled middorsally . 10
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- 12. Class Gastropoda Mostpopular class of molluscs. Consists of snails and snail-like creatures. Specific characteristics: They have a dorsally located shell (often coiled). Shell is composed of single piece that is univalved and spirally coiled. The shell has elongated form and hollow cone spirally coiled around central axis called columella. A single coil of shell round the axis is called whorl. Dextral shell: aperture in right side and coling anticlockwise. Sinistral: left aperture coilingclockwise. 12
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- 14. Class Bivalvia (Pelecypoda) Specific Characteristics: They have no head. They have two shells held together by powerful muscles. They have a ventrally located foot that sticks out between the two valves. Habitats include: marine and freshwater. They tend to burrow into soft mud or sand or attach to rocks or other shells. Large gills are used for respiration and filter feeding. Food is trapped by mucus on the gills and moved by cilia. Water enters and exits through siphons. 14
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- 16. Class Cephalopoda Containsthe largest molluscs. Specific Characteristics: May lack a shell (like an octopus) Shell may be reduced to a stiffening rod (like the squid or nautilus) The foot is highly modified to form a group of tentacles around the mouth. They are found in deep and shallow waters along many coasts. Squids & Nautilus are free-swimming and move very quickly. Octopuses are found among rocks or crawling on the bottom of the ocean. Complex brain, two lateral eyes, excellent eyesight. 16
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