888 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY, 
leading into a rapidly narrowing oral cavity, which 
contracts into the narrower width of the oesophagus (figs. 
1 and 36. Al.e¢.Z). This latter is a tubeieaboms 
din. long in P. maximus, dorso-ventrally compressed and 
leading upwards and posteriorly to the stomach. The 
opening is not at the anterior end of the stomach, but a 
hittle 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 
oceurs 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 width, but 
since 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, Al.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 erescentic, anterior, left lateral depression (fig. 38, 
