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Phylum mollusca

  2.  Def:-  The coelomate animal phylum whose members possess a head – foot, visceral mass, mantle cavity . Most molluscs also possess a radula and a calcareous shell.
  3. CHARACTERISTICS  Body of two parts: head foot and visceral mass.  Mantle that secretes a calcareous shell and covers the visceral mass.  Shell: Monovalvular or bivalvular or absent or internal  Bilateral symmetry.  Mantle cavity function in excreation,gas exchange, elimination wastes products.  Open circulatory system in all except in one class cephalopod.  Protostome characteristics.  Sense organs: Eyes, statocyst, sense or touch, smell and gustatory.
  4.  Reproductive system: Dioecious or monoecious, gonads with one or two ducts.  Fertilization: External or internal.  Excretion: By nephridia or kidney.  Habit and habitat: Terrestrial, aquatic, marine or fresh water. the body of molluscs has three main reagion 1) head-foot:- is elongted with an anterior head containing the mouth ,nervous nd sensory stucture and elongated foot used for locomotion.
  5.  Visceral mass contains the organs of digestion,cirulation,reproduction and excretion.
  6.  Mental: attatches to viscral mass,may secreate a shell that overlies the mantle.  The shell of mollusc is secreated in three layers  Outer layer called periostracum .it is secreated from mantale,s outer margin nd protein layer.  Prismatic layer is middle layer and contains CaCO3 and organic materials. Thickest layer……  Nacreous layer is inner that is form from thin sheet of calcium carbonate.its secreation thickns the shell
  7.  Mantle cavity: between the mantle and foot is space called mantle cavity.  Functions:  gas exchange  excretion  elimination of digestive wastes  release of reproductive system
  8.  Radula : the mouth of molluscs possess a resping structure called radula.  Consists of rows of teeth.
  9.   Classification of phylum Mollusca  Class Aplacophora  Class Polyplacophora  Class Monoplacophora  Class Scaphopoda  Class Bivalvia  Class Gastropoda  Class Cephalopoda  Class Caudofoveata
  10.  Cephalopoda: there are approximately 650- 700 species  includes octopus,squid, cuttlefish.  Most complex form of molluscs  Anterior end modified to forms tentacle's or armed used for capture prey, attachment,locomotion.  Shell:  external shell is present in nautilus  In some shell is absent as in octopuses  In cuttlefish shell called cuttlebone
  11.  Digestive system The digestive tract consists of three parts:  esophagus, which may contain a crop; stomach, which mashes food; and caecum , where most digestion and absorption occur.  Locomotion in cephalopods :  is accomplished mainly by jet propulsion  Octopus can use their arms to "walk,“or crawl  Reproduction:  dioecious  A female possess single ovidut. one tantacles of male cephalopods called hectocotylus modify to form spermatophor transfer.
  12.  Nervous system:  Cephalopods possess well-developed nervous systems and complex sensory organs.  Economic importance; Many species of squid and octopus are eaten.  Nautilus shells are often used decoratively.  The internal shell of a cuttlefish, or cuttle bone, is sold in the pet trade as a calcium source for birds.  Giant cephalopods such as squid and octopuses are also a great source of sea-monster folkore. . Class Gastropoda: includes the snails and slugs.
  13.  Shell: Gastropods are characterized by "torsion," a process that results in the rotation of the visceral mass and mantle on the foot.  Mostly have a single, usually spirally coiled shell .  Body form: Gastropods have a muscular foot which is used for “creeping" locomotion in most species modify for swimming.  Most gastropods have a well-developed head that includes eyes, 1-2 pairs of tentacles, and a concentration of nervous tissue
  14.  Reproduction:-Gastropods are dioecious, and some forms are hermaphroditic.  Marine species have veliger larva. torsion occure at that stage.  Monoplacophora: Living representatives of this Class were not discovered until 1952,  Paleozoic fossil monoplacophorans had been known for some time  11 species are known.  Most live at great depths and all are marine  . Monoplacophorans are regarded as ancestral to bivalves, gastropods, and cephalopods.
  15.  Class aplacophora:  250 species approxemetely  Lack shell and crawl on ventral surface.  Ladderlike nervous system  Gonochoric/gonochoristic/dioecious (sexes separate)  • Simultaneous hermaphrodite  • Sexual  • Fertilization  o external  o internal  Aplacophorans move via cilia through or on substrate.  • Motile  feed on microorganisms and detritus  E.g andrew
  16. CLASS SCAPHOPODA  There are approximately 900 species of Scaphopoda  Tooth shells or tusk shells  Scaphopods are all marine species  Conical shell at both ends.  Mostly buried in substrate with head and foot at downward apex of shell upward  Incurrent and excurrent water enters nd leaves mantle cavity at the apex.  Sexes are sparate trochophore and viliger larvae produced.
  17. CLASS POLYPLACOPHORA  Polyplacophorans include about 600 extant species  Nervous system resembled with apolacophora.  A chiton has calcareous spicules in the mantle.  The head is reduced, and lacks eyes and tentacles.  Sexes are separate.  Feed on attached algae.
  18. CLASS BIVALVIA  There are approximetely 1500 species of bivalvia.  characterized by possessing two shells secreted by a mantle .  The oldest part umbo…  Pearl formation.  Most marine bivalves go through a trochophore stage before turning into a free-swimming veliger larva .  a glochidium attaches to fish gills.  The glochidia larvae of some freshwater mussels can be serious parasites of fish.