Mollusca

• Phylum Mollusca


  A large phylum (2nd largest after Arthropods) of invertebrate animals known as molluscs, about 85,000 living species are recognized (Chapman, 2009)
• Mollusc is derived from the French mollusque, which originated from the Latin molluscus, from mollis, soft
• The largest marine phylum, comprising about 23% of all named marine organisms. There are numerous freshwater and terrestrial molluscs
• Molluscs are highly diverse in size, anatomical structure, behaviour and in habitat

  
• The phylum is typically divided into 9 taxonomic classes, of which two are entirely extinct

  • Monoplacophora
  • Polyplacophora
  • Scaphopoda
  • Gastropoda
  • Bivalvia
  • Cephalopoda
  • Aplacophora
• Molluscs


  • Three distinct body zones: a head-foot, visceral mass (contains well-developed organs of digestion, excretion & reproduction) and a mantle
  • All modern molluscs have mantle (or pallium) at the dorsal part of the body wall

  
• Molluscs

  • Two pairs of main nerve cords (Ruppert & Barnes, 2004), pedal and visceral nerve cords
  • Internal organs (visceral mass): concentrated in the dorsal part of the body
• Radula


  A rasping "tongue" with chitinous teeth, unique to molluscs, used for scraping food particles, absent in Class Bivalvia but present in the remaining five classes, internal in Class Scaphopoda and cannot extend beyond the body
• Muscular foot


  For locomotion, small (only at front end) in Class Scaphopoda, broad in the rest, but modified into arms in molluscs in Class Cephalopoda
• Molluscs


  • Large digestive caeca: absent in Class Scaphopoda, but present in the rest
  • Metanephridia ('kidneys') for excretion: are small & simple in Class Scaphopoda, but large & complex in the rest
  • True coelom is present, fluid-filled, develops within mesoderm, functions as a hydrostatic skeleton, internal organs are suspended in coelom
  • Unsegmented and bilaterally symmetrical
• Respiration in Molluscs


  Most molluscs have only one pair of gills, or one unpaired gill for gaseous exchange, gills have cilia, many freshwater species and majority of terrestrial species have pallial lung (lung sack)
• Feeding in Molluscs


  Molluscs with radula use it to scrape bacteria and algae off rocks, molluscs secrete slimy mucus, to which food sticks, beating cilia drive the mucus towards the stomach, bivalves: the mouth is equipped with labial palps to collect detritus from its mucus
• Digestion and Excretion in Molluscs

  Digestive tract is complete and ciliated, with a mouth, anus and complex stomach, undigested materials are compressed and packaged in solid form, discharged through anus into the mantle cavity and carried away in water currents, excretion: a pair of nephridia, tubular structures that collect fluids from the coelom, nephridia empty into mantle cavity
• Circulation in Molluscs


  Molluscs (excluding cephalopods) have open circulatory system, blood does not circulate entirely within vessels, cephalopods have a closed circulatory system, blood flows in vessels and hearts
• Nervous system in Molluscs

  Molluscs have two pairs of main nerve cords (3 in bivalves), the visceral cords serve internal organs & pedal ones serve the foot, most molluscs have a head with eyes, and all have a pair of sensor-containing tentacles that detect chemicals, vibrations and touch
• Reproduction in Molluscs


  Some molluscs have separate sexes & rely on external fertilization, all produce eggs, fertilized egg may hatch to form larvae or miniature adults, some molluscs use internal fertilization and are hermaphrodites, snails reproduce by inseminating each other in pairs to internally fertilize their ova, snails can produce up to 100 eggs, it takes approximately 2-4 weeks for eggs to hatch
• Class Monoplacophora


  All non-gastropod molluscs with a single shell or all single-shelled molluscs with serially repeated units, they have pseudometamerism (false segmentation) of bilaterally symmetrical repeated organs and muscles, habitat: they live only in deep ocean at depths below 180 metres (590 ft), they have as many as six nephridia, sexes are separate with ovaries or testes connected to either the third or fourth pair of kidneys
• Class Polyplacophora


  Chitons are small to large marine molluscs, about 940 extant species are recognized, largest chiton (up to 33 cm in length), chiton has a dorsal shell composed of eight separate shell plates or valves (for protection) joined by girdle, plates overlap at front and back edges and permit the chiton to flex upward when needed for locomotion over uneven surfaces, can slowly curl up into a ball when dislodged from underlying surface, habitat: chitons live worldwide, in cold water, warm water and in the tropics on hard surfaces, such as on or under rocks, or in rock crevices, exclusively and fully marine unlike bivalves and gastropods, most inhabit intertidal or subtidal zones, and do not extend beyond the photic zone, a few species live in deep water (6,000 m or 20,000 ft), chitons are eaten in many islands in the Caribbean, predators include humans, seastars, crabs, lobsters and fish
• Class Scaphopoda


  Tusk shells or scaphopods are shelled marine molluscs ranging from 0.5 to 15 cm in length, live in the subtidal habitat and seafloor sediment, feed on microscopic organisms, some supplement their diet of zooplankton with vegetable matter, also feed on detritus, have no eyes, reproduction: separate sexes, external fertilisation and eggs hatch into larvae

  
• Class Gastropoda


  "gastropod" (stomach-foot) is derived from Ancient Greek words (gastér, gastr-) "stomach", and (poús, pod-) "foot'', about 80% of all known mollusc species are gastropods: snails and slugs, the most diverse class with 60,000 to 80,000 living snail and slug species, habitat: sea, freshwater & terrestrial, reproduction: separate sexes in many marine gastropods, most land gastropods are hermaphrodites
• Class Bivalvia


  Marine and freshwater molluscs with laterally compressed bodies enclosed by a shell in two hinged parts, include clams, oysters, mussels and scallops, used as food, pollution control, making of jewels, many live in intertidal zone, gills have evolved into ctenidia, specialised organs for feeding and breathing, majority are filter feeders
• Class Cephalopoda


  Cephalopods are exclusively marine animals with bilateral body symmetry, prominent head surrounded by a set of arms or tentacles modified from the primitive molluscan foot, the longer two (tentacles) are for capturing prey, the shorter four pairs (arms) are used to hold and manipulate the prey, they have suckers that help to hold onto the prey, examples include decapods (e.g. cuttlefish & squid) and octopods, reproduction: the male has a sperm-carrying arm (hectocotylous arm), mating occurs head to head, cephalopods produce eggs, most live for one to two years, reproducing and then dying shortly thereafter