1) Sound monophyletic phylum, 2nd most diverse (>50,000 species)
a) seven classes of living species (3 major, 4 minor)
b) widespread in all major environments: marine (all classes), freshwater (gastropods and bivalves), terrestrial (gastropods)
c) although modern species from the major classes live in many habitat types, these classes largely reflect radiations in three (epibenthic- gastropods, infaunal- bivalves, pelagic- cephalopods)
2) The success of the phylum reflects the versatility of it's body plan:
a) Soft, fleshy body has four largely independent regions: the mantle (often with a shell), the head, the foot, the viscera b) two unique and functionally important traits (ctenidium='gill' and radula) have also permitted a variety of feeding specializations
3) The mantle is a thin layer of tissue that covers most or all of the body (like an umbrella) and produces new shell by accretionary growth at the edge; shell form varies greatly among the classes
4) An extensive mantle cavity supports many functions: respiration (all), feeding (some), locomotion (some), brood chamber (some)
a) the mantle cavity has different forms in different taxa
b) a unique ciliated gill (bipectinate ctenidium) pumps water and plays an important role in suspension feeding in some groups
c) an individual may have one to many ctenidia depending on taxon
5) All but one class possesses a unique feeding structure: the radula
a) flexible ribbon studded with many rows of chitinous teeth
b) complex muscles move ribbon back and forth over an odontophore
6) Two fluid-filled cavities lie within the mesoderm: coelom and hemocoel
a) coelom arises via schizocoely and is greatly reduced (only surrounds heart, gonad, and kidneys)
b) hemocoel is extensive: blood is pumped by heart through vessels to spongy tissues ("open" circulatory system- not fully enclosed by epithelium so capillary networks are reduced or absent)
c) hemocoel functions as a circulatory system and hydraulic skeleton
7) Spiral cleavage yields a "molluscan cross"; most have a trochophore larva; some pass through a veliger stage; all have a complete gut