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CLASSIFICATION OF MOLLUSCA

CLASSIFICATION OF 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.

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CLASSIFICATION OF MOLLUSCA

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  1. CLASSIFICATION OF MOLLUSCA

  2. 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.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. They are 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. Eg: Mussel, Oyster,Snail,Cuttlefish,Chiton

  8. Molluscs are classified into seven classes according to their symmetry, shell, mantle, gills, nervous system and radula. 1.Gastropoda 2.Amphineura 3. Solenogastres 4. Pelecypoda 5. Scaphopoda 6. Cephalopoda 7. Monoplacophora

  9. Class 1: Gastropoda Characters 1. Molluscs with distinct head bearing tentacles and eyes. 2. A ventral flat muscular foot . 3. Univalve shell which is spirally coiled. 4. The mantle which secretes the shell is a continuous fold enclosing the visceral mass,which is also spirally coiled. 5.The disposition of the internal organs shows asymmetry which is well marked in the nervous system Classified in to 2 subclasses

  10. Sub class I:Streptoneura (Prosobranchiata) 1.Presence of shell & operculum 2.The head bears a single pair of tentacles. 3.The mantle cavity opens anteriorly. 4.The ctenidium lies infront of the heart. 5.Sexes mostly separate & the males have usually a copulatory organ.

  11. Order:1 Aspidobranchiata (Diotocardia) Example:Patella(Limpet) Patella

  12. Order 1: Diotocardia 1.Bilateral symmetry are seen in the presence of two ctenidia,two kidneys & two auricles. 2. Ctenidia are bipectinate & free anteriorly. Example:Patella(Limpet) • Order 2: Pectinibranchiata (Monotocardia) Examples: Pila globosa( Apple snail) Turritella Xancus Conus Trochus Murex

  13. Turritella Xancus

  14. Conus Pila globosa

  15. Murex Trochus

  16. Characters of Monotocardia 1.Bilateral symmetry is completely lost. 2.There are presence of only a single auricle ,ctenidium & a single kidney. 3.Ctenidium is monopectinate and attached to the mantle throughout its length. 4. Sexes are separate.

  17. Sub Class II: Euthyneura 1.Visceral loop is untwisted. 2. Mantle cavity is displaced far backwards. 3. Head bears two pairs of tentacles. 4.No operculum. 5.Sexes are united.

  18. Order 1: Opisthobranchiata Example :Aplysia(Sea hare)

  19. Characters: 1. All marine with aquatic respiration 2. Mantle cavity is idely open & occupies a posterior position. 3. Shell shows a tendency to reduction. 4.Ctenidium also disappear.

  20. Order 2: Pulmonata Example : Limnaea (Pond snail),Achatina

  21. Mantle cavity is reduced. • The shell is often reduced. • Operculum is never present. • They are terrestrial but some live in fresh water.

  22. Class 2: Amphineura It includes the chitons which live in the sea between tide-marks, generally attached to rocks. They are sluggish animals feeding on sea weeds. Their body is elongate and dorso-ventrally compressed. A flat sole like foot extending along the whole length of the body serves for creeping & adhering to the substratum.

  23. CHITON

  24. A calcareous shell is present on the dorsal side & it is composed of eight overlapping plates. There is no distinct head. Body is bilaterally symmetrical & covered by a mantle. The mouth & anus are at the opposite ends of the body. • There are numerous pairs of bipectinate ctenidia lying on either side of the body in the mantle groove between the mantle & foot . Eyes and special sense organs are absent. Sexes are separate.

  25. Class 3 : Solenogastres eg. Neomenia Worm like degenerate marine, without mantle, foot or shell. Body is covered with a skin containing calcareous spicules.Ctenidia are absent. Alimentary canal is straight. Hermaphrodite animals.

  26. Neomenia Dorsal view Ventral View

  27. Class 4: Pelecypoda Bilaterally symmetrical with a shell of 2 valves , wedge shaped foot. Distinct head is absent, no tentacles, body is laterally compressed and covered with mantle. The gills are lamellate.Coelom is reduced. The alimentary canal is coiled. Horny jaws are absent. Eyes are absent. Sexes are separate.

  28. Sub Class 1:Protobranchiata The gills or ctenidia are single pair in the form of a plume with a central axis and a row of flattened filaments on either side.The foot has a flat sole adapted for creeping. Eg: Solen

  29. SOLEN

  30. Sub Class 2: Filibranchiata A pair of lamellate gills,in which the filaments are reflected .Twoadductor muscles are present.The foot has a byssus glandwhich secrets byssus threads for atttachment. Eg: Mytilus(sea mussel), Lithodomus

  31. MYTILUS

  32. LITHODOMUS

  33. Sub Class 3: Pseudolamellibranchiata GillsLamellate.Only one adductor is present.foot is feebly developed.shell are unequal. Eg:Ostrea(edible oyster)

  34. OSTREA (EDIBLE OYSTER)

  35. Sub Class 4: Eulamellibranchiata GillsLamellate.shell equivalved.Both adductors are well developed. order (i) Order(ii) Intergripalliata Sinupalliata Eg: Lamellidens ,Unio Eg: Cardium,Pholas

  36. LAMELLIDENS

  37. UNIO

  38. CARDIUM

  39. PHOLAS

  40. Sub Class 5: Septibranchiata Gills in the form of a perforated muscular partition extening between the visceral mass and the mantle. Eg:Poromya.

  41. Class 5: Scaphopoda(Tusk shells) It forms Small group of Mollusca,consisting of 3 genera. All marine,living in the sand at great depths.Mantle folds are fused ventrally, so as to form a tube,shell secreted by the mantle.No distinct head.Sexes are separate.Eyes are absent. Eg:Dentalium

  42. DENTALIUM

  43. Class 6:Cephalopoda It is an active swimmer .It feeds on animals like crabs,prawns,small molluscs and marine worms.It has no external shell,But internal shell is present. • Sub Class I:Dibranchiata It has the mouth is surrounded by eight or ten arms bearing suckers on their oral faces.The funnel is complete tube .An ink sac is present. The Dibranchiata are divided into two orders, Decapoda and Octopoda

  44. Order 1:Decapoda There are ten arms ,two of which (tentacular arms) are longer than the rest and retractile into special pouches.The arms bear pedunculated suckers. Eg:Sepia ,Loligo

  45. SEPIA

  46. Order 2: Octopoda They have only eight arms.The body is short and rounded aborally.Suckers have no horny rings.The shell is absent. Eg:Octopus

  47. OCTOPUS

  48. Sub Class:Tetrabranchiata Body is enclosed in a spirally coiled many chambered shell,lying all in one plane. 2 orders: • Nautiloidea – Eg: Nautilus It is nocturnal and lives at moderate depths swimming or crawling with its arms. 2.Ammonitoidea – Extinct

  49. NAUTILUS

  50. CLASS MONOPLACOPHORA: • Gr. Monos means one; plax means plate; pherein means bearing. • 1. The class consists of a small group of living species which are marine. • 2. They possess a single, bilaterally symmetrical shell which varies in shape from a flattened, shield-like plate to a short cone. • 3. The repetition of gills, retractor muscles, auricles and nephridia is a characteristic feature of living species.

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