57 



^4/. c. 2'), and a little behind this is another almost 

 circnlar posterior left lateral depression leading into a 

 short caecum (fig. 38, Al. c. 2"). Into the first there open 

 four to seven ducts ("bile ducts") from the digestive gland, 

 and into the second there are about three openings. These 

 ducts are situated on the side walls in P. niaximus, and are 

 very numerous. The number is, however, variable and 

 difficult to make out, as in some cases ducts may join 

 before reaching the stomach. Un the left side they all 

 open into the two depressions mentioned. On the right 

 side there is an antero-posterior groove into which as 

 many as ten duets may open. The stomach is usually 

 found to contain the remains of vegetable matter. The 

 walls are lined by a gelatinous-looking substance, found 

 also in other lamellibranchs and known as the " fleche 

 tricuspide " ; this will be considered later along with the 

 crystalline style, of which it is in all probability a part. 



The epithelium of the stomach is for the most part 

 a smooth layer, but on the left side the lining of the 

 posterior wall of the anterior depression has a number of 

 delicate ridges separated by grooves, leading from the 

 openings of the ducts into the main cavity of the stomach. 

 It is on the right side, however, that this grooved 

 epithelium is best seen, and it forms quite half of the 

 wall, extending from the whole width of the crescentic 

 depression to the opening of the intestine, towards which 

 the grooves are all directed. The grooves are not 

 separate and parallel along their whole length, but open 

 into one another as the intestinal opening is reached. 

 This epithelium of the stomach rests on a basement 

 membrane of almost structureless connective tissue. 

 Outside this there is a muscular la^'er made up of fibres 

 running in different directions, but chiefly transversely. 

 External to the muscle layer, and between it and the 



