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Phylum Mollusca – Phylum and Class Characteristics

Phylum Mollusca – Phylum and Class Characteristics . Phylum Characteristics: mantle secretes shell ; muscular foot ; radula (belt of rasping teeth) Overview of Diversity and Class Characteristics

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Phylum Mollusca – Phylum and Class Characteristics

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  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

  3. Fig. 16.24

  4. Fig. 16.33

  5. Fig. 16.32

  6. Fig. 16.12

  7. Fig. 16.2

  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)

  10. Fig. 16.3

  11. Fig. 16.5

  12. Fig. 16.6

  13. Fig. 16.13

  14. Fig. 16.16

  15. Fig. 16.23

  16. Fig. 16.22

  17. Fig. 16.4

  18. Fig. 16.17

  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

  24. Fig. 16.14

  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

  27. Fig. 16.37

  28. Fig. 16.38

  29. Fig. 16.39

  30. Fig. 16.40

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