68 OYSTER BOTTOMS IN MATAGORDA BAY. 



vegetable substance at all, but consists of the closely aggregated egg 

 cases of a snail-like mollusk, Purpura. It is found in dense masses 

 upon the oysters and shells of Half Moon Reef, the growth being 

 about one-half inch long, extremely tough and leathery, and of a 

 rich crimson color. It is objectionable in itself as interfering with 

 culling, and the mollusk to which the eggs give rise is reputed to drill 

 the oysters, although the author has never been able to satisfy him- 

 self absolutely of the truth of this assertion. 



FOOD OF THE OYSTER. 

 CHARACTER OF FOOD AND MANNER OF FEEDING. 



The food of the oyster consists mainly of microscopic plants, prin- 

 cipally of the kind known as diatoms, together with a small number of 

 microscopic animal organisms, Infusoria, some of which so closely re- 

 semble plants that their exact status is still a matter of dispute among 

 naturalists. Diatoms, a number of species of which are illustrated 

 (pis. xi, xii, and xiii), vary greatly in shape and size, but all resemble 

 one another in the interesting character of encasement in a siliceous 

 or glassy shell, usually beautifully sculptured, and nearly all of them 

 have the power of independent movement. Most of them exhibit a 

 golden brown coloration, unequally distributed, but there are a few 

 blue-green species. Prorocentrum, one of the so-called animal or- 

 ganisms referred to above, is an equally minute green body, propelling 

 itself by means of a taillike lash, and it, too, is sometimes inclosed in 

 a capsule, which, however, is not siliceous in structure. Though 

 both diatoms and Infusoria are capable of motion by their own 

 powers, their movements are too feeble to transport them any con- 

 siderable distance and are only sufficient to raise them above the bot- 

 tom, where, however, the organisms are brought within the action of 

 tidal currents, which become the chief agency of transportation and 

 bring about their general distribution. 



The oyster feeds upon these minute bodies by straining them 

 through its sievelike gills from the same water which it utilizes in 

 respiration, and it passes them on to the mouth through feeble cur- 

 rents set up by the lashing of innumerable microscopic bristles which 

 clothe the gills and the neighboring organs. These currents are the 

 only means by which the oyster can reach out into the water sur- 

 rounding it and bring to itself the food there supplied, and so weak 

 are they and so limited in their radius of action that the supply avail- 

 able to each individual oyster would be soon exhausted were it not 

 constantly replenished by tidal currents bringing new bodies of food- 

 laden water within reach. In still water, therefore, the oyster is 

 able to obtain less food than in flowing water of the same fertility. 



