52 LAND AND FRESH-WATER SHELLS. 



backwards to the mid-ventral region, where it redoubles upon 

 itself, and after running for a little distance pierces the anterior 

 end of the pericardium, from whence it runs as a straight rectum 

 through the ventricle of the heart, ending by an anus on the 

 dorsal side of that posterior portion of the supra-branchial chamber 

 which is sometimes spoken of as the cloacal chamber. The 

 rectum has a well-marked typhlosole running its whole length, the 

 function of which is probably to increase the absorptive area. 

 In the walls of the intestine of most Lamellibranchs, or in an 

 appended caecum, a peculiar crystalline style is often found, the 

 formation and function of which have been made the recent 

 subjects of research by Hazay. It is an albuminoid body, which 

 probably acts as a stopper to the pylorus. Its formation takes 

 place from spring to autumn, and is generally completed by 

 October by the differentiation of a gelatinous mass in the 

 stomach, which is apparently food-material. The style is, how- 

 ever, absorbed during the winter months, and the question naturally 

 occurs to us whether or not we must look upon it as reserve 

 nutrient material. The cells lining the intestinal tract are ciliated. 

 The liver, digestive gland, or hepato-pancreas lies between 

 the anterior adductor muscle and the pericardium, and on either 

 side of the stomach, into which it pours its secretion through 

 several ducts. It secretes trypsin and a diastatic ferment, and 

 the extract of the gland is active in either an acid, a neutral or 

 an alkaline medium. There are no calcareous cells, the cells 

 present being granular and ferment cells. 



The Organs of Circulation. — The heart consists of two 

 auricles and a pear-shaped median ventricle, situated in a peri- 

 cardium, which communicates with the glandular portion of the 

 nephridium by several reno-pericardial apertures. The pericardial 

 cavity is pierced by the intestine, the ventricle by the rectum. 

 Between each auricle and the ventricle there is an auriculo-ven- 

 tricular valve consisting of pocket-shaped segments, which only 

 admit the blood from the auricle into the ventricle. From the 

 anterior end of the ventricle the anterior aorta arises and runs 

 along the dorsal surface of the rectum ; from its posterior end the 

 posterior aorta arises and runs along the ventral surface of the 

 rectum. The anterior aorta passes into the visceral mass behind 



