72 



The epithelial cells lining the ascending- puvtiun of 

 the alimentary canal contain numerous granules like food 

 granules, and the same green tint is present here also. 

 It would seem from this that the pigment of the digestive 

 gland is derived directly from the food taken into the 

 alimentary canal. 



The rectum, after passing through the pericardium, 

 continues its course over the adductor muscle under the 

 connective tissue sheath. It does not run in the median 

 line, but crosses toM^ards the left side. At about the level 

 where the circular muscles of the mantle edge are inserted 

 to the shell the rectum leaves the adductor, and for its 

 last centimetre is free. The anus is surrounded by a 

 prominent lip. 



BLOOD SYSTEM. 



The blood system which has been worked nut in 



Fecial in((.riniu>i is very complete, and, like the vascular 



system of lamellibranchs in general, is entirely closed, so 



that water cannot enter the circulation directly. Tliere 



is, moreover, no communication between the heart and 



the pericardial cavity, and this does not contain blood. 



The system comprises true vessels, mainly arteries, which 



are large and easily followed in injections, but the venous 



system is much more lacunary in character. The amount 



of blood is considerable, and when the adductor muscle 



has been cut, on opening the shell as much as 25 c.c. of 



hlood have been obtained from one specimen. The blood 



is a colourless fluid slightly thicker than water, witli a 



strong saline taste. As is general for aquatic 



invertebrates, the osmotic concentration of the blood is 



practically the same as that of the sea water in wliich 



the specimen has been living, and small changes in the 



concentration of the outer medium are followed bv tlie 



