Phylum Mollusca, p. 1
PHYLUM MOLLUSCA
EVOLUTIONARY PLACEMENT (review on p. 62)
Three phyla have the same developmental patterns:
Spiral Cleavage - which is also determinate
Protostomes - mouth develops first with invagination
Schizocoelous - the mesoderm develops by a splitting off of endoderm cells
Large & diverse group snails, slugs, clams, octopus, squids, chitons (some pictures, p. 179)
CHARACTERISTICS (p. 175)
Bilateral symmetry, unsegmented, head present or absent
Organ systems well developed in a visceral mass: digestive system (complete) respiratory (gills or lungs) circulatory (open = heart w/o complex vessels) nervous (numerous nerves, no centralized brain) excretory (cluster of nephridia)
Movement typically by a muscular foot (p. 181)
Dorsal body wall has an extension into a mantle (pp. 182)
1) portion modified into gills on some
2) secretes the shell in most species
Have a true coelom, but it's typically restricted to area around heart (p. 181)
Most have modified mouth parts (fig. 10.4, p. 181) radula for scraping food in most groups
Larval stage of a trochophore larva (fig. 10.6, p. 183) top-shaped & has a row of cilia around body

Phylum Mollusca, p. 2
CLASSIFICATION
Eight classes recognized based on characteristics of:
1) presence of shell & it's form -- absent, coiled, straight, plates
2) mechanism of feeding -- scraping, filter feeder, predatory
3) presence of head & it's form
4) foot shape and movement
CLASS POLYPLACOPHORA - chitons (p. 184-185) marine, typically on shallow rocky coasts flat, 2-30 cm long (1/2 - 12") shell of 8 limy & articulating plates use foot mostly for attachment, move very little use radula to scrape algae off rocks breath via gills along mantle
CLASS SCAPHOPODA - tusk shells (p. 185-186) well know for their long skinny shell, tusk-like commonly live buried in sand or mud of deeper water, up to 6000 meters! size = 2.5-25 cm long foot used to burrow in sand obtain food with tentacle-like extension tail end of tusk above surface for breathing generates water current through the mantle cavity
CLASS GASTROPODA - snails, slugs and related species (p. 186-191) over 40,00 species known, also includes the limpets variation in shell:
1) none = slugs
2) one, uncoiled = limpets
3) one, coiled = snails (left or right handed) shells may also have a cover plate = operculum habitat varies widely: marine, freshwater, terrestrial lands forms restricted by humidity (must keep moist)

Phylum Mollusca, p. 3 all have a radula for feeding most eat algae by scraping some are scavengers some even predatory, feeding on clams! respiration typically by gills typically some with lungs (highly vascular area of mantle) circulatory & nervous systems typically well developed include a number of tactile organs (inc antennae)
CLASS BIVALVA - bivalves, clams (pp. 192-196) two half shells which fit together oldest part sticks out = umbo two adductor muscles close shells pearl production: sand between mantle & shell no head, no radula, little or no cephalization are filter-feeders, generate current through body generate currents by cilia on gills two siphons present, incurrent & excurrent movement minimal, initiated by muscular foot some move by closing shells quickly (scallops) gills allow for both gas exchange and feeding mucous secreted by labial palps to trap food food moves through stomach & intestines anus located near excurrent siphon circulatory system with a well developed heart blood pumped through two vessels to other organs blood returns into the pericardium (the coelom) sexes are typically separate reproductive cells released through excurrent siphons fertilization external larvae free swimming later glochidium larva attach to gills of fish, live as parasites for several weeks

CLASS CEPHALOPODA - cephalopods (pp. 196-199) squid, octopus, nautilus, cuttlefish all are marine commonly 2-3 m long; giant squid is largest invertebrate head well developed with eyes & radula also have arms/tentacles off head foot modified into a siphon for movement octopus also uses arms to move nervous systems well developed --> brain, eyes both squids & octopi have ink glands secrete dark pigment (melanin) mostly for defense while escaping
Three distinct groups of cephalopods:
1) nautilus: with coiled and chambered shell older chambers gas filled tentacles all alike poorly developed circulatory system
2) squids: with internal flexible shell mantle flattened to fins smaller arms + larger tentacles well developed circulatory system
3) octopus: with no shell mantle not modified all arms alike well developed circulatory system
Phylum Mollusca, p. 4