Marb 435 1st Edition Lecture 10 Current Lecture Mollusk 1 Second largest phylum with regard to species diversity About 100 000 described extant species About 35 000 described extinct species good fossil record due to calcareous exoskeleton Seven classes Classes Aplacophora worm mollusks Monoplacophora rare mollusks Polyplacophora Chitons Scaphopoda tusk mollusks Gastropods snails and slugs Bivalvia clams Cephalopoda octopus and squid Synapomorphies Dorsal epithelium forms aMantle which secretes calcareous spicules or one or more shells Radula for feeding Ventral body wall musculature develops into locomotory or clinging foot Mollusks have These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Radula rasping tool with chitinous teeth that occur in all mollusc classes except bivalves Coelom restricted to thepericardium space around the heart and gonads Open blood vascular system with dorsal heart and aorta opening into the hemocoel Hemocoel Blood filled main body cavity surrounding most organs Spiral holoblastic cleavage All mesoderm develops from the mesoblast i e the 4d micromere in the 64 cell stage Coelom develops from solid mesodermal blocks that split up to form cavities schizocoely Trochophore Larvae Typical larval form of phyla Mollusca Annelida and Sipuncula in modified form in some nemerteans Can be modified replaced by other larval forms or skipped entirely May be planktotrophic or lecithotrophic Class Polyplacophora Chitons Shell composed of eight overlapping plates or valves Strongly dorsoventrally depressed Sometimes with spicules in the girdle Head poorly developed no tentacles or eyes Most common in the intertidal zone About 800 extant species Most scrape algae and other organisms from the surface of rocks and shells Have a very long radula compared to other molluscs Some macroherbivores feed on seaweeds Some carnivorous species Nervous system and Sensory organs Simple low degree of cephalization Ladder like many commissures aesthetes sensory cells that extend through the shell plates Class Aplacophora a k a worm molluscs No shell Body covered with calcareous spicules Body shape elongated in anterior posterior direction Mantle cavity at the posterior end Cleavage 1 Spiral 2 Holoblastic 3 Unequal Class Monoplacophora Single shell with anteriorly curved apex Small and flattened Only about 20 extant species in 3 genera One species occurs in 200 m depth all others are from 1800 7000 m Rare today but abundant in the fossil record Class Scaphopoda tusk mollusks Exclusively marine Live in sandy or muddy sediment with the anterior end pointing down Elongated in anterior posterior direction Shell open at both ends Use foot for burrowing Capture food using captacula Have no ctenidia heart or circulatory system