226 PEOFESSOn F. M. BALFOUR. 



support. Its walls are formed of five layers, viz. from without 

 inwards. 



(1) A peritoneal investment. 



(2) A layer of longitudinal fibres. 



(3) A layer of circular fibres, amongst which are numerous 

 nuclei. 



(4) A connective-tissue layer supporting (5) a layer of fairly 

 columnar hyaline epithelium, bounded on its inner aspect by 

 a cuticle continued from that of the pharynx. In front it 

 passes insensibly into the pharynx, and beyond the region 

 where the dorsal walls of the pharynx have clearly com- 

 menced, the ventral walls still retain the characters of the 

 oesophageal walls. The oesophagus is vertically oval in front, 

 but more nearly circular behind. Characteristic of the oeso- 

 phagus is the junction of the two sympathetic nerves on its 

 dorsal wall (fig. 16). These nerves cannot be traced far beyond 

 their point of junction. 



The Stomach. — The next section of the alimentary tract is 

 the stomach or mesenteron (fig. 6). It is by far the largest part 

 of the alimentary tract, commencing at about the second pair 

 of legs and extending nearly to the hind end of the body. It 

 tapers both in front and behind, and is narrowest in the middle, 

 and is marked off" sharply both from the oesophagus in front and 

 the rectum behind, and is distinguished from both of these by 

 its somewhat pinker hue. In the retracted condition of the 

 animal it is, as pointed out by Moseley, folded in a single 

 short dorsal loop, at about the junction of its first with its 

 second third, and also, according to my observations, at its 

 junction with the rectum ; but in the extended condition it is 

 nearly straight, though usually the posterior fold at the junction 

 of the rectum is not completely removed. Its walls are always 

 marked by plications which, as both Moseley and Grube have 

 stated, do not in any way correspond with the segmentation 

 of the body. In its interior I have frequently found the 

 chitinous remains of the skins of insects, so that yye are 

 not justified in considering that the diet is purely vegetable. 

 It lies free, and is, like the remainder of the alimentary 



