Zoology 250 Lecture 16
MOLLUSCA I: Overview
(Mollusca study images)
- 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 & 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 supports many functions: respiration (all), feeding (some), locomotion (some), brood chamber (some)
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
- 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 & forth over an odontophore
- 6) Have two fluid-filled cavities within mesoderm: coelom & hemocoel
a) coelom is greatly reduced (surrounds heart, gonad, & kidneys)
b) hemocoel is extensive: blood is pumped by heart through vessels to spongy tissues ("open" circulatory system, no capillaries)
c) hemocoel functions as a circulatory system & hydraulic skeleton
- 7) Spiral cleavage yields a distinct embryo with a "molluscan cross"; most have a trochophore larva; some pass through a veliger stage; all have a complete gut
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