SEA-FISHERIES LABORATORY. 359 
Al.c.2'), and a little behind this is another almost 
circular 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?. mavtmus, 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. On 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 ducts 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 1s 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 layer made up of fibres 
running in different directions, but chiefiy transversely. 
External to the muscle layer, and between it and the 
