OYSTERS AND METHODS OF OYSTER-CULTURE. 287 



In most regions which have been investigated the plants constitute 

 by far the most important item of diet, usually over 90 per cent of the 

 food contents of the stomachs being composed of vegetable matter. 

 Of this diatoms are the chief constituents, and to a certain extent the 

 food value of any given oyster region may be measured by the quantity 

 of these minute plants which it is capable of producing. 



Diatoms are numerous both in species and individuals, and all possess 

 two interesting peculiarities : They are incased in a siliceous or flinty 

 box and they possess the power of locomotion, the first permitting their 

 ready identification in the stomach contents and the second aiding in 

 their distribution. More or less regular diurnal migrations of swarms 

 to and from the surface of the water take place with the variations in 

 the light. During sunlight they rise from the bottom, and are then 

 readily transported by the currents, again settling down as darkness 

 comes on. They feed and grow in size most actively during the day, but 

 multiply in number principally at night. Diatoms are important, not 

 only in fattening the oyster, but they also have a i^rofound influence 

 upon its flavor and color. 



The oyster is said to feed mainly during flood tide, opening its shell 

 at that time to admit the influx of water with its contained organisms. 

 Investigation by Dr. Bashford Dean showed that the stomachs were 

 practically foodless in the morning, contained most food at midday, 

 and a somewhat reduced quantity at evening, thus suggesting that 

 feeding was most active during intense daylight. 



Dr. Dean remarks : 



This suggestion, as to the feeding habits of the oyster, is not a surprising one 

 when we remember that it is during the strongest sunlight than diatoms, as plants 

 keenly sensitive to the sun, are most active and are known to migrate in iloating 

 clouds from the bottom of the surface. 



As is mentioned in the section relating to the anatomy of the oyster, 

 the water drawn into the mantle cavity by the action of the cilia is 

 filtered through the rectangular openings in the gills into a chamber or 

 tube lying above each gill, whence it passes backward and out of the 

 shell in a current dorsal to the entering stream. The particles of food 

 in the inflowing stream become entrapped in a sticky mucus covering 

 the gills, and, together with this mucus, in j)art, are carried in a steady 

 stream toward the mouth, the motion being imparted to the mass by 

 the rhythmic action of the cilia. The palps and mouth are also 

 ciliated, which insures the continuance of this current into the stomach, 

 where the food ])articles undergo digestion. A very considerable pro- 

 portion of inert matter, sand, mnd, etc., of no Lutrient value passes into 

 the alimentary tract along with the food, the oyster having no means 

 of making selection. 



The temperature, depth, and density of the water have considerable 

 effect upon the food supi)ly. In clear, warm weather the amount of 

 food matter is increased by the natural multiplication of the minute 



