Phylum Mollusca – Phylum and Class Characteristics

1. Phylum Mollusca – Phylum and Class Characteristics • Phylum Characteristics: mantle secretes shell; muscular foot; radula (belt of rasping teeth) • Overview of Diversity and Class Characteristics • Class Polyplacophora: chitons; shell composed of 7-8 limy plates; common herbivores in tide pools • Class Gastropoda: snails; most diverse of marine animals (approx. 70,000 extant species); most with single shell; some terrestrial species (pulmonates) • Class Cephalopoda: octopuses, squids, and relatives; shell reduced or absent; largest brain among invertebrates; beak-like jaws; exhibit complex behaviors; varied diet includes other molluscs • Class Bivalvia: clams, scallops, mussels, oysters; shell with two valves; lack radula and head; most marine; foot reduced or used for digging; include shipworms (cause damage to wooden piers) • Class Scaphopoda: tusk/tooth shells; burrowers; feed on detritus and meiofauna with ciliated tentacles

2. Fig. 16.1

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8. Fig. 16.42

9. Body Structures in Molluscs • Head-Foot Portion • Head with sense organs (eyes, antennae), mouth, and tentacles • Foot: muscular, provides suction and movement; modified (bivalves  digging; cephalopods  arms and tentacles) or vestigial (ex. mussels attach to substratum with byssal threads) • Visceral Mass: internal organs typically covered by shell • Ciliary tracts include gills (some w/ lungs) and mantle (produces shell) • Open circulatory system (excl. cephalopods); metanephridia; nervous system with multiple ganglia and interconnecting nerve cords • Shell: one or two valves; reduced in opisthobranchs (ex. Aplysia); nudibranchs and octopuses lack (often with defensive chemicals) • Three layers: outer periostracum (organic), prismatic layer, and inner nacreous layer (with “mother-of pearl”) • Radula: a protrusible belt of rasping teeth • Used for grazing in chitons and many gastropods; used for drilling in predatory snails and octopuses; modified tooth  harpoon in cone snails; bivalves filter-feed via siphons (radula absent)

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19. Fig. 16.28

20. Fig. 16.34

21. Larval Development in Molluscs • Larval Stages (some molluscs with direct development, ex. Cephalopods hatch from egg with adult form) • Trochophore: shared larval stage with annelids; free-swimming stage emerges from egg; develops into adult stage (chitons) or into veliger stage (gastropods and bivalves) • Veliger: free-swimming larval stage with foot, shell, and mantle • Glochidium: specialized parasitic larvae of freshwater clams; attach to specific fish hosts for weeks • Gastropod Development • Torsion: process where gastropod larval body twists such that anus and mantle cavity opens near head (causes some fouling of gills) • Spiral coiling: process where gastropod shell coils during larval development (occurs concurrently with torsion) • Evolutionary questions regarding functions of torsion and coiling • Two mantle cavities (ex. chitons)  one cavity (torsion) • Weight distribution of shell modified by coiling • Ancestral molluscs with bilateral symmetry, anterior/posterior ends

22. Fig. 16.35

23. Fig. 16.7a, 16.8, & 16.36

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25. Fig. 16.15

26. Diversity and Behavior of Cephalopods • Diversity and Characteristics: siphon allows jet propulsion; brain with multiple lobes; all members are active, marine predators • Ammonites: predominate ocean predators in Mesozoic (extinction at end of the Cretaceous); chambered shells are common fossils • Nautiloids: five extant species (Nautilus spp.); two pairs of gills; 60-90 tentacles without suckers; gas-filled chambers in shell allow neutral buoyancy; simple eye with constricted pupil • Coleoids: one pair of gills; eight arms; squids and cuttlefish with reduced shell (pen), fins, and two tentacles; octopuses lack shell; complex eyes with lens and retina; diversity includes Humboldt and giant squids (Architeuthis), mimic octopus, and many bizarre deep-sea species • Behaviors: chromatophores control color changes (courtship, crypsis); ink released as defense; many toxic (blue-ringed octopus deadly); squids with giant neurons (control rapid escape response); octopuses capable of observational learning

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30. Fig. 16.40