IIIb. Class Bivalvia Clams, oysters, mussels, scallops. All marine or freshwater Second largest class All have 2 shells. Locomotion 1. Clams/mussels- muscular foot for burrowing 2. Oysters- adults are sessile 3. Scallops- “clap” shell . B. Shell Features 1. Shell structure
1. Shell structure
a. Periostracum- thin outer layer of protein
b. Prismatic layer- thick mid layer of calcium carbonate
c. Nacreous layer- inner layer; secretes nacre which forms pearls in some.
2. Umbo- swollen area near anterior dorsal end of clam; oldest part of shell.
3. Hinge ligament- holds two shells together
4. Growth lines- wider lines indicate more growth/better feeding season.
5. Anterior and Posterior adductor muscles- very strong muscles for opening or closing shell.
1. Filter feeders
2. Food/water enter incurrent siphon.
3. Food swept into mouth by 2 pairs of palps
4. Esophagus, stomach, intestine, anus
5. feces/excess water leave thru excurrent siphon.
1. Oxygenated water enters incurrent siphon.
2. Gills have tubes where water & blood are close together. Gases exchange by diffusion across the membrane.
3. Deoxygenated water leaves thru excurrent siphon.
2. Heart and short vessels similar to gastropods.