228 PROFESSOR F. M. BALFOUR. 



rectum is somewhat folded. The usual arrangement of the 

 parts is that shown in fig. 6, where the hind end of the stomach 

 is seen to be bent upon itself in a U-shaped fashion, and the 

 rectum extending forwards under this bent portion and joining 

 the front end of the dorsal limb of the U. The structure of 

 the walls of the rectum is entirely different to that of the 

 stomach, and the transition between the two is perfectly sudden. 

 Within the peritoneal investment comes a well-developed mus- 

 cular layer with a somewhat unusual arrangement of its layers, 

 there being an external circular layer and an internal layer 

 formed of isolated longitudinal bands. The epithelium is fairly 

 columnar, formed of granular cells with large nuclei, and is 

 lined by a prolongation of the external cuticle. It is raised 

 into numerous longitudinal folds, which are visible from the 

 surface, and give a very characteristic appearance to this part 

 of the alimentary tract. The muscular layers do not penetrate 

 into the epithelial folds, which are supported by a connective 

 tissue layer. 



Nervous System. 



The central nervous system consists of a pair of supra- 

 oesophageal ganglia united in the middle line, and of a pair of 

 widely divaricated ventral cords, continuous in front with the 

 supra-a3Sophageal ganglia. 



It will be convenient in the first instance to deal with the 

 general anatomy of the nervous system and then with the 

 histology. 



Ventral Cords. — The ventral cords at first sight appear to 

 be without ganglionic thickenings, but on more careful exami- 

 nation they are found to be enlarged at each pair of legs (PI. 

 XV, fig. 8). These enlargements may be regarded as imper- 

 fect ganglia. There are, therefore, seventeen such pairs of 

 ganglia corresponding to the seventeen pairs of legs. There 

 is in addition a ganglionic enlargement at the commencement 

 of the oesophageal commissures, where the nerves to the oral 

 papillffi are given ofi" (PI. XVIII, fig. 22 or. g.), and the region 

 of junction between the oesophageal commissures with the supra- 



