MOLLUSCA (IV): DIVERSITY- Bivalves etc.
(Mollusca study images;
6) Class BIVALVIA (mussels, oysters, scallops, clams, etc.); 2nd most diverse class (~25,000 marine and freshwater spp)
a) body plan highly modified for: a) burrowing and b) particle-feeding:
b) two shells (valves) joined by flexible ligament (tension or compression); mantle edge fuses to shell margin and links to 2 adductor muscles; may be isomyarian (same size) or anisomyarian (different size)
c) most produce byssal threads as larvae; some still do as adults
d) gill form is critical to understanding the evolution of bivalves. One pair of ctenidia resides inside a large mantle cavity; they may be either:
both have compound latero-frontal cilia for particle capture
e) foot is highly modified for burrowing (except attached or swimming species); burrowing involves both shell and foot
f) sexes are separate; gametes spawned freely into water; most have free-swimming trochophore and bivalved veliger larval stages
g) classification still debated; 4 Subclasses: Paleotaxodonta, Pteriomorpha, Heterodonta, Anomalodesmata (based on shell hinge teeth, and form of ctenidia and adductor muscle; see lab guide)
7) Class SCAPHOPODA (tusk shells) ~250 burrowing marine spp
Copyright (c) 2013 by A. Richard Palmer. All rights reserved.
(revised Jan. 3, 2013)