554 MARINE PRODUCTS OF COMMERCE 



pensated in the oyster by a highly developed chemical sense. The organs by which 

 the oyster tastes the water are tiny finger-like tentacles set in two rows on the edge 

 of each mantle. When the oyster opens its shell the tentacles slightly protrude 

 beyond the edges of the valves and guard the entrance to the shell cavity. Pres- 

 ence of a toxic or irritating substance makes them contract. The stimulus, if 

 strong enough, is conveyed through the mantle to the powerful adductor muscle 

 which contracts and closes the shell. In this way access of harmful substances is 

 prevented and the oyster remains safe inside its calcareous home. The tentacles 

 are also sensitive to light and react to sudden changes in illumination. 



The principal function of the mantle which surrounds the body of the oyster 

 is the formation and repair of shell. The growth of shells, both in length and in 

 thickness, continues throughout the life of the oyster, but the rate of growing 

 slows down with age. 



Opening and Closing of Shell. The powerful muscle attached to both valves 

 controls the closing and opening of the shell. When it contracts, the shell remains 

 tightly closed; when it relaxes the springlike action of a hinge, a dark, elastic 

 band of organic substance, by which the two valves are joined together forces 

 them apart. Shell movements of an oyster can be easily recorded on paper mounted 

 on a rotating drum. Study of such records reveals to the biologist important facts 

 regarding the effect of environment on the behavior of the oyster. In the presence 

 of small concentrations of harmful substances the normal muscular curve is al- 

 tered, closing and opening of valves becomes irregular, and the time the oyster 

 remains closed and not feeding lengthens. Such observations are usually made in 

 a study of the eflFect of pollution on oysters. 



Respiration and Feeding. Like many other aquatic animals the oyster possesses 

 giUs, a highly complex organ, which is concerned primarily with respiration. 

 However, its function in the bivalve mollusks underwent considerable change and 

 the gills became important parts of the feeding mechanism as well as an organ 

 of respiration. 



The gills in the oyster are located immediately under the mantle, two on each 

 side; they may be compared to a fine sieve covered with hairlike cilia. The holes 

 of the sieve, so-called ostia, are surrounded by relatively large cilia, which beat 

 inward and produce a current of water which passes through tubelike structures 

 inside the gills and is expelled through the cloaca. 



The exposed sides of the gills are covered by small cilia which beat along the 

 surface and push toward the edge of the gill the minute unicellular algae and 

 other particles which are carried in with the current of water and settle on the 

 gills. In feeding and respiration the oyster filters large volumes of water from which 

 it obtains its food and oxygen. Material settled on the surface of the gills is en- 

 tangled in mucus secreted by special cells and is gradually carried toward the 

 mouth and ingested. Methods are available now (Galtsoff, 1946) for recording 

 the amount of water filtered by the oyster. In cold water the rate of pumping 

 slows down and completely stops at temperatures below 42° F (5.6° C). It im- 

 mediately increases as the water temperature is raised and reaches its maximum 

 at about 77-80° F (25-26.7° C). 



Toxic substances discarded into water with industrial wastes retard the rate 

 of pumping and greatly interfere with the feeding (Galtsojff et al., 1947, Chip- 

 man and GaltsoflF, 1949). Records made in various laboratories under different 



