Chp 9

• Mollusca


  Large phylum of invertebrate animals
• Around 85,000 extant species of mollusks are recognized
• Mollusks are the largest marine phylum, comprising about 23% of all the named marine organisms
• Numerous mollusks also live in freshwater and terrestrial habitats
• Modern mollusks


  • Have a mantle with a significant cavity used for breathing and excretion
  • Presence of a radula (except for bivalves)
  • Structure of the nervous system

  
• Mollusks are coelomates, but the coelom tends to be small
• The main body cavity is a hemocoel through which blood circulates; their circulatory systems are mainly open
• The phylum is typically divided into 9 or 10 taxonomic classes, of which two are entirely extinct
• Classes of mollusks studied

  • Amphineurians (aplacophora, monoplacophora, polyplacophora)
  • Scaphopods
  • Gastropods
  • Bivalves
  • Cephalopods

  
• Cephalopod mollusks


  Among the most neurologically advanced of all invertebrates
• The giant squid or the colossal squid is the largest known invertebrate species
• The gastropods (snails and slugs) are by far the most numerous mollusks in terms of classified species, and account for 80% of the total
• Class Aplacophora


  • Marine, worm-like mollusks, without shell
  • Body is cylindrical, 1-15 cm long
  • Mouth opens at the anterior end, anus opens in a posterior cloaca
  • Lack nephridia, gonoducts assume excretory functions
  • Heart consists of one auricle and one ventricle
• Aplacophora class is divided into two orders: Solenogastres and Caudofoveata
• Class Monoplacophora


  • Marine mollusks (3-30 cm), found at great depth (3000m)
  • Circular shell consists of one piece
  • Ventral foot is flattened and surrounded by a pleural groove containing 5 pairs of gills
  • Nerve system consists of a cerebral commissure related to two cerebral ganglia
  • Heart is composed of two pairs of auricles and one pair of ventricles
  • Sexes are separate, two pairs of gonads related with the 3rd and 4th pairs of nephridia

  
• Class Polyplacophora


  • Marine mollusks, found in littoral zone
  • Shell consists of 8 articulated plates
  • Foot is flattened and surrounded by pleural cavity containing many pairs of bipectinate gills
  • Heart consists of two auricles and one ventricle
  • Sexes are separate, gonads are fused and have two gonoducts

  
• Class Scaphopoda


  • Marine mollusks, body (0.5-25 cm) is enclosed within a elephant tusk-like shell opened at both ends
  • Digestive system consists of a mouth surrounded by tentacles called captacula, pharynx with radula, stomach related to a digestive gland, and intestine
  • A single pair of nephridia (metanephridia) opens near the anus, gills are absent
  • Nerve system consists of two cerebral ganglia, two pedal ganglia, and two pleural ganglia
  • Sexes are separate, there is a single gonad related to the left nephridium

  
• Gastropoda subclasses


  • Prosobranchia
  • Opisthobranchia
  • Pulmonata
• Class Bivalvia


  • Mantle is divided into two lobes that secrete the bivalve shell
  • Without head, nor radula
  • Development passes by a glochidium larva (freshwater) or trochophore and veliger larvae (marine and some freshwater)
  • Gills may be bipectinate or folded, cerebral and pleural ganglia may be separate or fuse

  
• Cephalopoda subclasses


  • Tetrabranchia (Nautiloidea)
  • Dibranchia (Coleoidea)
• Tetrabranchia (Nautiloidea)


  • Possess two pairs of gills, two pairs of nephridia, two pairs of auricles and a large number of suckerless tentacles
  • Compound eye lacks lens, ink gland is missing
  • Body has a shell composed of many chambers, vital parts occupy the newest and largest chamber
  • Siphuncle extends through the septa of the chambered shell and secretes gas into empty chambers to make it buoyant
• Dibranchia (Coleoidea)


  • Possess one pair of bipectinate gills, one pair of nephridia, and one pair of auricles
  • Lenses of the compound eyes and the ink gland are present
  • Tentacles are suckered, ink gland contains dark liquid which can be discharged to aid escape
• Dibranchia (Coleoidea) orders


  • Octopoda
  • Decapoda
• Octopoda


  • Possess eight tentacles, without shell
• Decapoda


  • Possess 10 tentacles, shell is reduced and usually covered by the mantle