36 SMITHSONIAN MISCELLANEOUS COLLECTIONS \ OL. 94 



the lips of the anus. The colon is the most strongly musculated part 

 of the intestine, and, when in a state of contraction, it often appears 

 as a very short connective between the distended ileal and rectal sacs. 



The posterior part of the rectum is provided with fan-shaped dorsal, 

 lateral, and ventral dilator muscles arising on the body wall and in- 

 serted on the proctodaeum in line with the longitudinal muscle bands 

 of the latter (fig. i6A). The dorsal dilators (286) arise medially 

 on the tergum of the tenth abdominal segment (fig. 14), and their 

 spreading fibers are inserted on the rectum along the lines of the latero- 

 dorsal longitudinal muscles. The lateral dilators (fig. 16 A. 2po) arise 

 laterally on the tenth tergum (fig. 14). and are inserted in line with 

 the lateral longitudinal muscles of the rectum. The ventral dilators 

 (fig. 16 A, 264) arise in the male at the bases of the anterior apodemes 

 of the ninth abdominal sternum (fig. 12). in the female on the 

 apodemes of the ovipositor (fig. 17 B), and are inserted in line with 

 the lateroventral longitudinal muscles of the rectum. 



The structure of the inner wall of the proctodaeum of Dissostcira 

 Carolina has been described and figured by Tietz (1923), who shows 

 that the several parts of the intestinal tube present characteristic his- 

 tological differences. Mewed internally, it is seen that the Malpighian 

 tubules open into 12 pockets of the pyloric region (fig. 16 B, g), and 

 that the pockets are somewhat overlapped anteriorly by a circular fold 

 of the enteric wall (/). The proctodaeal intima lines the Malpighian 

 pockets and is reflected to the edge of the overhanging fold. It is 

 clear, therefore, that the crest of this fold (/) is the true line of 

 separation between the mesenteron and the proctodaeum, and that 

 the Alalpighian tubules arise from the proctodaeum. Between the 

 Alalpighian pockets the wall of the proctodaeal pylorus forms 12 broad, 

 padlike thickenings (/?), which are crossed externally by the sphincter 

 muscle (f), and which, therefore, may constitute collectively a pyloric 

 closing apparatus between the stomach and the colon. Posteriorly the 

 pyloric pads are narrowed and are either continued as well-marked 

 folds on the wall of the ileum ( /) , or they are broken up into numer- 

 ous small folds, according to the degree of tension in the ileal wall. 

 The external longitudinal muscle bands of the ileum {e) lie between 

 each alternate pair of internal folds. When the folds are accentuated 

 by contraction of the proctodaeal muscles, they extend posteriorly 

 through the colon to the rectum. According to Tietz (1923) the proc- 

 todaeal intima has a thickness of .008 mm in the ileum, and of .012 mm 

 in the colon. 



The inner wall of the rectal sac presents six long, flat, parallel thick- 

 enings lying between the external bands of longitudinal muscles, each 



