Zoology 250 (2002) MOLLUSCA: Diversity IV (Mollusca study images; click on tree to see full cladogram)

   Previous   Home   Next

• 1) Class CEPHALOPODA (nautilus, squids, octopus) ~500 marine spp

  a) includes largest, fastest, and most intelligent invertebrates; purely carnivorous (eat fish, crustaceans, molluscs)

  b) body plan modified for pelagic life; shell is reduced or absent in many; head & foot form tentacles & siphon

  c) extinct forms had well-developed shells with gas-filled chambers

  d) mantle is muscular & mantle cavity is large; jet-propulsion is achieved via antagonism of circular & radial mantle muscles

  e) impressive eyes, balance organs (statocysts) and nervous system (largest brain in invertebrates, giant axons & stellate ganglia)

  f) 2 pair (Nautilus only) or 1 pair (all others) of ctenidia lack cilia

  g) have a true 'closed' circulatory system lined by endothelium

  h) sexes separate; copulate with hectocotylus; unusual development

  i) three subclasses: I) Nautiloidea (1 genus); II) Ammonoidea (extinct); III) Coleoidea (Or. Decapoda, Or. Octopoda) (based on number of ctenidia & arms/tentacles, and on presence/absence of suckers)

Zoology 250 (2002) PHYLUM SIPUNCULA (peanut worms) (Sipuncula study images; click on tree to see full cladogram)

• 1) Geographically widespread; ~300 spp of marine deposit feeders

• 2) Their bizarre body plan includes:
  
  a) two coeloms lined by peritoneum (small- tentacles, large- trunk)
  b) a completely invertable head end! (the introvert)
  c) a crown of tentacles surrounding mouth collects food particles from adjacent surfaces (surface deposit feeding)
  d) an unusual, U-shaped gut (terminal mouth, mid-dorsal anus)

• 3) They are related to molluscs by several distinctive characteristics:
  
  a) molluscan cross (apparent in the 64-cell stage)
  b) an eversible scraping buccal organ in the larva; some think it may be related to the radula in gastropods
  c) a creeping muscular lip in the larva; some think it may be related to the creeping foot in benthic molluscs

Back to Zool 250 Home Page