718 NATURAL HISTORY OF AQUATIC ANIMALS. 



amount of absorbing surface very materially. The internal surface of the stomach is also very 

 much plicated; but here the folds are both large and conspicuous, with small folds often inter- 

 vening. There are neither annular nor longitudinal muscular fibers in the wall of the intestine ; 

 the sole motive force used in the propulsion of the ingested food appears to be exerted by the 

 ciliary covering which everywhere clothes the internal surface of the alimentary tract from the 

 mouth to the anus. 



It would appear that the intestine makes two complete bends upon itself at a very early stage 

 of embryonic life, according to the observations of Horst, long before it measures a ninetieth of 

 an inch in diameter. The development of the liver seems to be at first lateral and somewhat 

 ventral; an arrangement traces of which may still be noticed in cross-sections of the adult. 



The course of the intestine in the adult may be described as follows: 



The mouth is a wide opening between the upper median angles of the palpi; so wide, indeed, 

 that the animal can scarcely be said to have an oesophagus ; immediately follows the stomach, 

 which is seen to have very pronounced folds internally, with a generally transverse direction, but 

 two of these, which lie in a somewhat ventral position, are a pair of inward-projecting folds which 

 are themselves plicated. The intestine then follows an oblique course, downward and backward, 

 when it makes a sharp bend returning beneath the fioor of the pericardial space, passing obliquely 

 upward and forward, somewhat to the right and dorsal of the stomach, when it crosses exactly 

 over the mouth or very short gullet, passing downward to the left side of the animal, alongside 

 and a little to the lower side of the stomach, when it again turns upward and passes over the 

 pericardial space to end in the rectum just over the middle of the adductor muscle. The clusters of 

 hepatic lobules or follicles dip down into the folds of the walls of the stomach, but the liver does not 

 follow the course of the intestine proper, which is provided internally with a curious pair of longi- 

 tudinal and parallel folds, which project into the intestinal cavity and extend from the pyloric end 

 to very near the anus. The presence of these folds gives to the faecal matters their singular appear- 

 ance, which are not in the form of a cylinder as they leave the vent, but in the form of a tube with a 

 part of one side removed. Tracing the course of the intestine by sections is not the proper way; 

 they can be very easily dissected out for their entire length by means of the scissors and forceps. 



The systemic heart of the Oyster is that organ which serves to propel and redistribute the 

 colorless blood of the animal through the body for. its nourishment, and through the gills that the 

 blood itself may discharge into the water the poisonous gases with which it is loaded in passing 

 through the body, and receive a fresh supply of oxygen as fresh supplies of water pass through 

 the gills. The heart consists of three principal chambers ; the upper, largest, whitish and partially 

 divided by a median septum or partition, is the ventricle, and the two lowermost and smaller, 

 brownish paired chambers are known as the auricles. These three chambers which comprise the 

 heart of the Oyster lie in a crescent-shaped space, the pericardial space, just forward of the 

 adductor muscle. The ventricle is almost globular; its walls are made up of a delicate mesh work 

 of unstriped muscular fibers, which are so interlaced as to be altogether untraceable. From the 

 ventricle a great posterior and an anterior aortic vessel arises. These two vessels distribute the 

 blood to the posterior and anterior portions of the body of the animal, but soon divide into paired 

 vessels which traverse the mantle on either side both anteriorly and posteriorly, while one great 

 median branch passes forward over the stomach. The blood is really distributed soon after 

 leaving the main vessels, especially in the body through the spongy connective tissue spaces, as 

 already described, and is collected into a great ventral canal from which a large part of it passes 

 into the gills. From the four gills or branchial pouches the blood flows back to the ventriclea 



