Phylum Mollusca


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General characters and classification of phylum mollusca


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Slide 1: 

1 BEATIFUL SEA LIFE Dr. R. K Pillai, Dept of Zoology, Hindu College, Moradabad Pre-Medical Presentation Diversity in Living World Phylum Mollusca

General Characters : 

2 General Characters 1. Number of species about 93,000 (living) and 70,000 (extinct) (the second largest phylum in number of species, the first being Arthropoda). 2. The term Mollusca (L., mollis, soft) was originally proposed by Jonston (or Jonstonus) in 1650 for cephalopods and barnacles. 3. De Blainville (1825) altered the name Mollusca to Malacozoa which was not popular among scientists 4. Malacology is the study of Mollusca and Conchology is the study of shell of Mollusca . 5. Mollusca comprises some of the best known invertebrates like, snails, slugs, clams, squids and octopuses. 6. Molluscs are triploblastic, bilaterally symmetrical (or secondarily asymmetrical ), unsegmented, coelomate protostomes. 7. Coelom limited to small spaces around nephridia, heart and part of intestine. 8. Principal body cavity is a haemocoel (open circulatory system).

Numbers of species : 

3 Numbers of species Mollusca - Second largest phylum

Slide 4: 

4 Myxozoa Arthropoda Annelida Mollusca Lophophores Hemichordata Vertebrata Other pseudocoelomates Nematoda Porifera Ctenophora Cnidaria Placozoa Platyhelminthes Nemertea Ciliophora Sarcomastigophora Microspora Apicomplexa Mesozoa Echinodermata Crustacea Chelicerata Uniramia Other Chordata

Generalized Mollusc : 

5 Generalized Mollusc

Body Plan : 

6 Body Plan Mantle cavity Ctenidium Pericardial cavity Metanephridium Radula Gonad Stomach and digestive gland Foot

Slide 7: 

7 9. Heart lies in pericardial chamber and composed of separate ventricles and atria. 10. The body is covered by a skin fold mantle with shell glands which secrete calcareous spicules, shell plates or shells. 11. Most molluscs secrete a shell of calcium carbonate that protects and supports their soft tissues. 12. The body is organized into three general regions: head, foot and visceral hump. 13. The head bears the mouth and sense organs. 14. The muscular foot is primitively used for locomotion, crawling in snails and digging in mussels, but in squid it is modified into tentacles for capturing prey. 15. The visceral hump contains the digestive tract and other visceral organs. 16. Digestive system complete with a digestive gland or liver (hepatopancreas). 17. Molluscs typically employ a rasping organ called radula which is armed with rows of chitinous teeth.

Slide 8: 

8 18. The radula is protruded from the mouth and worked back and forth to rasp the food into fine particles. 19. Blood with amoebocytes, respiratory pigment is copper containing haemocyanin dissolved in plasma. 20. Respiration by ctenidia (gills) , lungs or both; sometimes direct. 21. Excretion by large, complex metanephridia (Kidneys, Organ of Bojanus). 22. Nervous system with paired ganglia, commissures and connectives. 23. Sense organs include eyes, statocysts and osphradia (a chemoreceptor to test chemical nature of water). 24. Reproduction sexual, dioecious or monoecious. 25. Fertilization is generally external, development direct or through free larval forms like trochophore, veliger, glochidium, etc. 26. Most molluscs are marine, some freshwater and few terrestrial. 27. Most molluscs are herbivores, but some (e.g., squid) are active predators.

Classification : 

9 Classification Molluscs are classified mainly based on shell and foot. There are seven classes of molluscs: 1. Aplacophora 2. Polyplacophora (Amphineura) 3. Monoplacophora 4. Scaphopoda 5. Gastropoda (Univalvia) 6. Pelecypoda (Bivalvia) 7. Cephalopoda

Class I Aplacophora (Gr., a, without; plak, plate; phoros, to bear) : 

10 Class I Aplacophora (Gr., a, without; plak, plate; phoros, to bear) 1. Also called solenogasters. 2. Benthic, marine, vermiform molluscs. 3. No shell, epidermis secretes aragonite (calcareous) spicules or scales. 4. Mantle cavity rudimentary, without eyes, tentacles or nephridia. 5. Surface dwellers on corals and other substrates and are carnivorous, frequently feeding on cnidarian polyps. 6. Approximately 120 species (Neomenia, Chaetoderma, Falcidens, Scutopus, etc.)

Class II Polyplacophora (Gr., polys, many; plak, plate; phoros, to bear) : 

11 Class II Polyplacophora (Gr., polys, many; plak, plate; phoros, to bear) 1. Commonly called chitons, inhabitants of hard substrates in intertidal shallow marine water. 2. Head reduced, body flattened, elongated, with broad ventral foot. 3. Without eyes and tentacles. 4. Mantle forms thick girdle that border shell plates. 5. About 1000 described species (Choriplax, Chiton, Katharina, Mopalia, Tonicella, etc.) Chiton

Class III Monoplacophora (Gr., mono, one; plak, plate; phoros, to bear) : 

12 Class III Monoplacophora (Gr., mono, one; plak, plate; phoros, to bear) Known only from fossils until 1952, when first living species Neopilina galatheae was discovered by Danish Galathea Expedition from 3,570 meters in the Pacific off Costa Rica. The most primitive mollusc having characters of annelids. 3. Only mollusca having segmentation (internal) or metamerism 4. A ‘living fossil’, a connecting link between annelids and molluscs.

Slide 13: 

13 5. With a single cap-like shell. 6. Mantle cavity shallow, around foot enclose 3-6 pairs of ctenidia, 2 pairs of gonads, 3-7 pairs metanephridia. 7. Head small, tentacles present only around mouth. 8. About 25 described species, most live at considerable depth (Neopilina, Monoplacophorus, Rokopella, Vema) .

Class IV Scaphopoda (Gr., scapha, boat; podos,foot) : 

14 Class IV Scaphopoda (Gr., scapha, boat; podos,foot) 1. Commonly called tooth shells or tusk shells. 2. The most distinctive characteristic is a tubular shell that opens at both ends. 3. The head and foot project from wider end of the shell. 4. With proboscis, radula and clubbed contractile tentacles (captacula). 5. Live mostly buried in the substrate feed on foraminiferans. 6. Nearly 900 living species (Dentalium, Pulsellum, Cadulus, Gadila, etc.).

Slide 15: 

15 Scaphopod Anatomy

Class V Gastropoda(Gr., gaster, belly; podos,foot) : 

16 Class V Gastropoda(Gr., gaster, belly; podos,foot) 1. Largest and most varied molluscan class comprising snails and slugs. 2. Terrestrial molluscs found only in this class. 3. Asymmetrical with single, usually spirally coiled shell. 4. During development, visceral mass and mantle rotate 90-180o on foot, a phenomenon called torsion. 5. Head with statocyst, eyes, 1-2 pairs of tentacles. 6. Foot large and flat. 7. About 70,00 living species (Pila, Aplysia, Doris, Patella, Limax, etc.). Snail

Slide 17: 

17 Snail Slug Limpet

Slide 18: 

18 Torsion in Gastropoda

Class VI Pelecypoda ( = Bivalvia ; = Lamellibranchiata)(Gr., pelekus, hatchet; podos, foot) (L. bis, twice; valva, leaf) : 

19 Class VI Pelecypoda ( = Bivalvia ; = Lamellibranchiata)(Gr., pelekus, hatchet; podos, foot) (L. bis, twice; valva, leaf) 1. The second largest molluscan class comprising clams, mussels, oysters , scallops and shipworms. 2. Laterally compressed, shell typically of two valves. 3. Head rudimentary, without eyes or radula. 4. Foot laterally compressed, usually without a sole. 5. Mostly filter feeding, usually dioecious, veliger or glochidium larva.

Slide 20: 

20 6. Mostly marine, few fresh water. 7. With nearly 20,000 living species of marine or fresh water molluscs (Unio, Anodonta, Mytilus, Teredo, Pecten, etc.) 8. Teredo is commonly known as ‘shipworm’; it is destructive to wood in sea water. 9. Pinctada vulgaris (Indian Pearl Oyster): Pearl is secreted by mantle on some external particles entangled in skin which causes irritation to animal. Pearl Oyster

Slide 21: 

21 Oysters

Pearl formation : 

22 Pearl formation Developing pearl Epithelium Shell Irritant lodged between shell and mantle Layers of nacre secreted around foreign material

Shipworms : 

23 Shipworms

Class VII Cephalopoda (Gr., kephale, head; podos, foot) : 

24 Class VII Cephalopoda (Gr., kephale, head; podos, foot) 1. Most specialized molluscs comprising cuttlefish, squids, octopods and nautiloids. 2. Most active molluscs, the foot is located on the head modified in the form of oral arms. 3. Head is distinct and large with well developed eyes and a ring of tentacles. 4. Part of the foot forms, a funnel-shaped siphon. 5. Locomotion is by expelling water in jet through siphon (Jet propulsion). Cuttlefish

Slide 25: 

25 6. Ink glands in some squids for offence and defence. 7. When the squid is attacked, it emits a cloud of inky fluid through its siphon. 8. This ‘smoke screen’ interferes with the visual and; chemoreceptors of the predator and thereby helps the squid to escape. 9. Cephalopods are exclusively marine and carnivorous. 10. Nautilus has an external shell, but in others the shell has been reduced and enclosed within the body or lost entirely. 11. About 900 species (Sepia, Loligo, Octopus, Nautilus, Spirula, Argonauta, etc.) Octopus

Squid : 

26 Squid Dorsal Ventral Posterior surface Right Left

Slide 27: 

27 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Cephalopod Eye

Nautilus : 

28 Nautilus Up to 94 tentacles No suckers Shell with many chambers Lives in outermost chamber

Trochophore Larva : 

29 Trochophore Larva A larval form common for annelids and molluscs

Phylum Mollusca : 

30 Phylum Mollusca Subphylum ACULIFERA Class Aplacophora Class Polyplacophora — chitons Chiton — Sea mica (Mail shell) Subphylum CONCHIFERA Class Monoplacophora Class Scaphopoda — tooth shells, tusk shells Dentalium— Tusk shell

Slide 31: 

31 Class Gastropoda — snails and slugs Patella — Limpet Fissurella — Keyhole limpet Trochus — Top shell Pila — Apple snail Crepidula — Slipper shell Cypraea — Cowrie Natica — Star shell Buccinum — Whelk Doris — Sea lemon Aplysia— Sea hare Turbo — Cat's eyes Vermetes— Worm shell

Slide 32: 

32 Nassa — Mud shell Conus — Cone shell Bulla — Bubble shell Pteropod — Sea butterfly Littorina — Periwinkle Philippa — Sundial shell Janthina — Violet shell Cyphoma — Flamingo tongue shell Bursa — Frog shell Columbella — Dove shell Helix — Land snail Limax — Grey Slug

Slide 33: 

33 Class Pelecypoda (=Bivalvia) — clams, mussels Unio — Freshwater mussel Mytilus — Sea mussel Dreissena — Zebra mussel Spondylus — Edible oyster Pinctada — Pearl oyster Pecten — Scallop Teredo — Shipworm Solen — Razor clam Tridacna — Giant clam Mya — Soft shell clam Pinna — Pen shell Anomia — Jingle shell

Slide 34: 

34 Class Cephalopoda (Siphonopoda) — squid, octopus, cuttlefish Sepia — Cuttlefish Loligo — Squid Octopus — Devilfish Spirula — Ram's horn Todarodes — Arrow squid Architeuthis — Giant squid

Slide 35: 

35 Architeuthis (Giant Atlantic squid) Largest of all invertebrates, length 20 meter including tentacles, weight exceeding 1 ton. The eyes are upto 20cm in diameter, the largest animal eye on record.

Slide 36: 

36 A true Objective Hand Book “A conscientious teacher of Zoology who has been guiding students for various competitions for last 30 years has put his best in this book” 11th Edition

Slide 37: 

37 End Thanks

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