304 Annals Entomological Society of America [Vol. XI, 



small longitudinal muscles intercalated between them. The 

 vessels do not extend along the wall of the rectum, but terminate 

 blindly at the extreme posterior end of the colon. 



It IS A7ery difficult to make out the exact course of the 

 vessels m the wall of the colon. They seem to extend posteriorly 

 as six parallel tubes, at first of slight diameter and almost 

 straight, but as the diameter increases, they become more and 

 more wavy, with larger and larger folds. It is this character 

 which makes the tubes appear so elongate in cross-section. 

 Toward the end of the colon, these undulatory folds are so large 

 that those of one series almost touch those of the adjacent 

 series, and thus they almost completely surround the wall of 

 the colon. The vessels seem to branch irregularly, the tubes 

 terminating blindly and separately in irregular ramifications, 

 just anterior to the strong circular muscles which appear 

 abruptly, and mark externally the beginning of the rectum. 



The writer has already figured the course of the vessels in 

 the wall of the colon (Woods, 1916, figure 4); and a series of 

 cross- sections of the colon showing the different conditions 

 described above is illustrated in figures 22-27. 



THE RECTUiM. 



Transition. The transition between the colon and the rec- 

 tum is the most abrupt in the whole course of the aHmentary 

 canal. The Malpighian vessels and the peritoneal sheath dis- 

 appear abruptly, and new circular muscles make their appear- 

 ance suddenly. The epithelial cells of the colon become flatter 

 and flatter, and the cell boundaries more and more distinct near 

 the rectum, but the type changes quite abruptly to the glandular 

 and eosinophile cells characteristic of the rectum. 



Intima. At first there are only minute wavy folds in the 

 intima and epithelium, but posteriorly these folds become 

 gradually more and more pronounced, and more and more 

 tooth-like in appearance, while the lumen becomes smaller and 

 smaller. The typical number of these "teeth" is probably six, 

 but they are far from regular, and there may be from five to 

 eight. As is to be expected, the primary and secondary intima 

 are continuous with the primary and secondary cuticula 

 respectively, of the body wall. There is no pigment in the 

 primary cuticula for a considerable distance beyond the rectal 

 invagination. 



