2G 



AA'Lieli reduces the lumen of the gut at that point to 

 less than one-third of its usual diameter (tig. 49). 

 Surrounding the adjacent portion of the hind-gut is a 

 strongly developed pyloric or intestinal valve, which is 

 formed by the circular muscles at that point becoming 

 enormously thickened {valve c. iti.). 



The hind-gut extends from the posterior border of 

 the fourth abdominal segment to the anus. It is lined by 

 a perfectly smooth chitinous cuticle, directly continuous 

 with that of the integument (cuf. in fig. 49), and under- 

 lying it is a cellular layer (h. g. cpith.), which rests 

 externally on a basement membrane {t. ij.). In the 

 anterior third of the hind-gut its epithelial layer is 

 greatly thickened, the nuclei are larger and cell- 

 boundaries are discernible ; over the rest of its course 

 this layer is relatively thin and of the nature of a 

 syucj^tium. Circular muscle fibres of the striped variety 

 are well developed in relation with the anterior half of the 

 gut, but they dwindle away as they proceed further 

 backwards. The cuticular and epithelial lining of 

 the gut is thrown into numerous folds and bulgings, 

 but the circular muscles do not follow their course 

 and large spaces are consequently left. (Plate III., 

 fig. o\). These spaces are in free communication 

 with the general body cavity between the muscles, 

 and usually contain blood corpuscles. In addition to 

 circular muscles, there is also a system of radial muscles 

 in relation with the hind-gut ; they fall into two groups, 

 the dilatores recti and the dilatores ani (fig. 49, dil. red. in. 

 and dil. an. m.). The dilatores recti chiefly arise from ■ 

 the middle section of the gut, where it is considerably 

 enlarged and forms a kind of rectal chamber. They 

 cross the cavity of the fifth abdominal segment in dorsal, 

 ventral, and lateral directions, and are inserted into the 



