IIIb.  Class Bivalvia
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IIIb. Class Bivalvia Clams, oysters, mussels, scallops. All marine or freshwater - PowerPoint PPT Presentation


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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

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  • 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

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.


C. Digestion/Feeding

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.


D. Respiration

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.

E. Circulation

1. Open

2. Heart and short vessels similar to gastropods.


F. Excretion

1. Nephridium

  • Nervous/Sensory System

  • 1. Ganglia concentrated in foot, esophagus, adductor muscle and mantle.

  • 2. Some have eyes for sensing light around shell rim.

  • 3. Osphradia- chemoreceptors


  • Reproduction

  • 1. Most are dioecious- external fertilization

  • 2. Sperm leave thru excurrent siphon

  • 3. Sperm brought in thru incurrent siphon of female.

  • 4. Some release trochophore larvae which develop into adults.


  • 5. Some release glochidia which look like miniature adults.

  • These are usually parasitic on fish.

  • Attach to fish gills & feed on blood.

  • When adult organs form, they drop off & take on filter feeding life style.



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