Zoology 250 Lecture 18
MOLLUSCA: Diversity II
(Bivalvia study images, Cephalopoda study images)
- 2) Class BIVALVIA (clams, oysters, scallops) 2nd most diverse class
a) body plan is highly modified for a burrowing, filter-feeding mode of life (body is laterally compressed & wholly enclosed inside mantle & shell; head & radula are absent!)
b) shell composed of 2 valves joined by a flexible ligament; mantle edge is fused to the shell margin & linked to adductor muscles
c) foot is highly modified for burrowing (except attached species); burrowing involves both shell and foot
d) 1 pair ctenidia; may be small & mainly respiratory (protobranch), or large and modified for filter feeding (lamellibranch): may be filibranch (ciliary junctions) or eulamellibranch (fused filaments)
e) most produce byssal threads as larvae; some still do as adults
f) classification is still hotly debated; 4 subclasses: Paleotaxodonta, Pteriomorphia, Heterodonta, Anomalodesmata (based on type of shell hinge teeth, form of ctenidia, adductor muscle form)
- 3) Class SCAPHOPODA, the 'tusk shells'
a) curious tubular shell; originates as two shells in larva that fuse
b) head is reduced, but have a well-developed foot for burrowing
c) lack ctenidia but possess a radula
d) are micro-predators; they capture foraminifera with captacula
- 1) Class CEPHALOPODA, 3rd major class (nautilus, squids, octopus)
a) includes most highly advanced, largest & fastest invertebrates; body plan modified for pelagic life
b) exclusively marine carnivores (eat fish, crustaceans, molluscs)
c) shell is reduced or absent; head & foot form tentacles & siphon
d) extinct forms had well-developed shells with gas-filled chambers
e) very sophisticated eyes and balance organs (statocysts)
f) 1 or 2 pair of ctenidia lacking cilia
g) have a true 'closed' circulatory system lined by endothelium
h) mantle is muscular and mantle cavity large; jet-propulsion is achieved via antagonism of circular & radial mantle muscles
i) three subclasses: a) Nautiloidea- 1 living genus; b) Ammonoidea-extinct; c) Coleoidea (Or. Decapoda, Or. Octopoda)
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(revised revised Mar. 28, 2000)