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 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
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