GAMMARUS. 345 



the former, just in front of the diverticula, is a flap of 

 epithelium, which, as Wrzeniowski suggests, may serve to 

 prevent what is passing along the alimentary canal from 

 returning, or it may prevent such material from entering 

 the diverticula. 



The hind-gut lies in the last three segments of the 

 abdomen (PL II.), and is sharply differentiated from the 

 mid-gut, though its ventral portion extends forward 

 further than the dorsal part, which ceases immediately 

 behind the origin of the posterior diverticula of the mid- 

 gut. Its Avails consist of (1) a membrana propria of much 

 greater thickness than that of the mid-gut, (2) a thick 

 layer of striated circular muscle fibres, (3) scattered 

 bundles of longitudinal muscle, (4) columnar epithelium. 

 The last is raised into broken longitudinal ridges, and 

 covered by a thick layer of cuticle which penetrates into 

 the hollows between them. These ridges become higher 

 towards the anus, and, probably, aid in passing along 

 excreta. 



The anus is a vertical slit, opening on the last seg- 

 ment, just below the telson (PI. II., an.) ; bands of 

 muscle connect its lateral walls with those of the last 

 body segment. 



Around the alimentary canal is a diffuse fat body 

 (PI. II., ad. t.), which is thickest round the reproductive 

 organ, but is also well developed around and 

 behind the oesophagus, and at the sides of the rectum. 

 It diminishes in fasting Amphipods, according to 

 Wrzeniowski (27). 



The "Vascular System. 



The circulation of the Amphipoda has been described 

 by Delage (3), who also summarised the work previously 



