324 FOSSIL OSTREID^ OF NORTH AMERICA. 



blood from all parts of the body, sends it to the lungs, receives it again 

 from the latter, to finally again send it off to the different parts of the 

 body. 



The food of the oyster is very various in character, as we find the 

 remains of small crustaceans, molhisks, larval worms, crustacean larvae, 

 rhizopods, diatoms, &c., besides inorganic earthy and siliceous mate- 

 rials, in the stomach. It is probably omnivorous, as M. Certes has hap- 

 pily expressed it ; the only condition which seems to be requisite in any 

 organic body to fit it for food for this animal is that it shall be small 

 enough to be passed through the wide but vertically much constricted 

 mouth and throat. The great bulk of the food of the oyster, however, 

 probably consists of minute marine larvae, infusorians, and diatoms, and 

 of these the latter, which are iilants of microscopic size, are found in the 

 greatest profusion. The diatoms have very delicately and beautifully 

 sculptured siliceous cases, which encase the endochrome or living matter 

 which becomes the food of the oyster. The empty siliceous tests of 

 diatoms are often found in great numbers, mixed among the earthy 

 matters and debris found in the intestine and stomach of the animal; 

 in fact mineral aiid indigestible remains of many thousands of individ- 

 ual plants may sometimes be found in a couple of grains of the faecal 

 matters. The soft organic matter contained in the stony cases of these 

 microscopic plants is rapidly dissolved by the digestive juices poured 

 out by the liver, leaving behind the indigestible tests or cases which 

 are carried out through the intestine along with the faecal matters. 



The most of the food of the oyster consists of minute, living, moving 

 beings; this is the case even with the diatoms, which are minute vege- 

 table organisms endowed with the powei* of movement. The same sys- 

 tem of minute filaments which clothes the gills and ministers to respira- 

 tion by sweeping the water through the latter in a constant current, is 

 also the principal agent concerned in carrying the food of the oyster, 

 which floats in the surrounding water, towards its mouth, as is indicated 

 by the arrows below the palps in Fig. 1. When it has once reached the 

 palps, the inner surfaces of which next to the mouth are jirovided with 

 narrow ribs or ridges, which are thickly covered with cilia, it is swept 

 down, or rather backwards, into the throat, from the point m, which 

 marks the position of the mouth. The throat itself, however, as well 

 as the entire alimentary canal, is clothed with vibrating cilia, which are 

 the active agents in sweeping the refuse of the food and other in gestae 

 through the alimentary tract to expel it at the vent v. 



In Fig. 2, Plate LXXIV, the course of the alimentary canal i is indi- 

 cated, together with its relations to the liver /, in longitudinal median 

 sect ion of the fore part of the body. This sketch was taken from an actual 

 dissection of a hardened specimen ; the right lobe of the outermost palp 

 and the soft parts of the right side have been removed in order to clearly 

 display the course and arrangement ot the alimentary canal . The paral- 

 lel folds or ridges are shown on the outer surface of the inner palp or lip 



