56 



leading into a rapidly narrowing- oral cavity, whicli 

 contracts into the narrower widtli of tlie oesophagus (figs. 

 1 and 36. Al. c. 1). This latter is a tube about 

 ■|in. long in P. maxiitms, dorso-ventrally compressed and 

 leading upwards and posteriorly to the stomach. The 

 opening is not at the anterior end of the stomach, but a 

 little further back and on the roof — rather to the right 

 side in P. opercularis. Both the oesophagus and stomach 

 are completely enclosed by the digestive gland, the 

 so-called liver (fig. 1, Dg.) The oesophagus is lined by a 

 thick epithelial layer, slightly marked by transverse 

 ridges, which has the same yellow-brown colour that 

 occurs so frequently, and which contrasts strongly with 

 the dark green digestive gland surrounding it. The cells 

 forming the epithelium are long, narrow and ciliated. 

 The height of the cells is many times the Avidth, but 

 8ince there is very little difference between the cells 

 lining the various parts of the alimentary canal, a fuller 

 description will be given later. Outside the epithelium 

 there is a basement layer of compact connective tissue, 

 and, outside this, looser connective tissue with transverse 

 and longitudinal muscle fibres, which is connected with 

 the strands that separate the tubules of the digestive 

 gland. 



The stomach (figs. 36 and 38, ^4/. c. 2) lies in the 

 midst of the digestive gland, but usually nearer to the 

 left side than the right. It is of very irregular, roughly 

 oval shape, with the longest diameter antero-posterior, 

 and with irregular folds and depressions breaking up the 

 wall into certain areas. Two of these on the left side 

 and one on the right are more important than the others 

 and occur with greater regularity. On the left side, not 

 far behind the level at which the oesophagus opens, there 

 is a crescentic, anterior, left lateral depression (fig. 38, 



