76 



BRANCH MOLLUSCA 



large adductor muscle. The eggs are deposited in the water. They 

 are very numerous. It has been estimated that one female will produce 

 from 9,000,000 to 40,000,000 eggs in a single season. The breeding season 

 is from May to August. If the eggs are not eaten by enemies or carried 

 away by currents, they sink to the bottom. After a few hours of develop- 

 ment the larvae swim to the surface. Multitudes of these larvae are de- 

 voured by surface-living fishes. The larvae (Fig. 56) swim by means of 

 cilia. In a few days the larvae or fry, as they are called, sink to the bottom 



Fig. 55. Food of South Carolina oyster. A few typical organisms 

 (x 225). Numbers 1 to 20 are diatoms. 1-5, Navicula (Bory); 6, N. 

 didyma (K.); 7, Pinnularia radiosa (?) (K. S.); 8, Amphora sp. (K.); 

 9, Pleurpsigma fasciola (E. S.); 10, P. littorale (S.); 11, P. strigosum (S.); 

 12, Actinocyclus undulatus (K.); 13, Coscinodiscus radiatus (E.); 14, 

 Cyclotella rotula (E.); 15, Synedra sp. (E.); 16, Diatoma sp. (De C.); 

 17, Cymbella sp. (Ag.); 18, Mastogloia smithii (Thw.); 19, Triceratium 

 alternans (Br. Bai.); 20, Biddulphia sp. (Gr.); 21, Grain of pine pollen 

 (Pinus rigida); 22, Foraminifera (Rotalia); 23, Zoospore (Ulva?); 24, 

 Spicules. (After Bashford Dean.) (From Moore, U. S. Com. of Fish 

 and Fisheries.) 



and attach themselves by the mantle-fold to some other oyster or to any 

 object with which they come in contact. It takes them from three to 

 five years to attain their growth. The blue crab (see Fig. 74, p. 101) is very 

 destructive to the young oyster. 



One of the greatest enemies of the oyster is the starfish (see p. 55). 

 Other enemies (Fig. 57) are boring snails, boring sponges, and internal 

 parasites. One little crab (Pinnothe'res) which lives in the mantle cavity 

 seems to be an example of symbiosis rather than a parasite; at least it does 

 not appear to harm the oyster. 



