1) A sound monophyletic phylum, 2nd most diverse (named species!)
a) seven classes of living species (3 major, 4 minor)
b) widespread in all major environments: marine- all classes, freshwater- gastropods & 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' & 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 and produces new shell by accretionary growth at the edge; shell form varies greatly among the classes
4) An extensive mantle cavity between the mantle and body may serve many functions (respiration, feeding, locomotion)
a) a unique ciliated gill (bipectinate ctenidium) pumps water and plays an important role in suspension feeding in some groups
b) an individual may have one to many ctenidia depending on taxon