L. E. Robinson and J. Davidson 
237 
The rectum commences as a funnel-shaped depression of the posterior 
portion of the ventral wall bf the stomach, immediately anterior to the 
bifurcation of the latter organ from which the postero-lateral coeca arise. 
It then continues, downwards and backwards, as a narrow cylindrical 
tube to the anterior portion of the under surface of the rectal sac, into 
which it opens at this point. In sections, the walls of the funnel-shaped 
upper extremity of the rectum are seen to be identical in structure with 
that of the stomach, but from this point downwards, the cells lose 
their individuality, the rectal wall here consisting of a finely granular 
homogeneous protoplasm in which is embedded a layer of crowded 
nuclei. The lumen of the tube, especially in its lower portion, is 
generally completely obliterated, and it does not appear to become 
patent until the final stages of the digestive process is reached. Not 
until the gastric epithelium has degenerated, do excretory products 
from the stomach appear in the rectal sac, and as the rectum is the only 
means of communication between the two organs, the excretory granules 
derived from the alimentary coeca and stomach, which do ultimately 
appear, mingled with the secretion of the Malpighian tubules in the 
rectal sac, must have traversed the rectum. 
The Rectal sac. 
Plates XIV, XV and XVI, fig. 6; Plate XVII, fig. 4, s. r. 
The rectal sac is a large vesicle, with delicate transparent walls, 
into which the rectum and the Malpighian tubules open, and from which 
a short canal communicates with the anus. It is usually rendered very 
conspicuous by reason of its opaque white contents. The body of the 
sac lies below the ovary in the female and the testis in the male; its 
lower surface is in contact with the ventral body wall. Just behind 
the anus, on either side of the series of postero-rnedian dorso-ventral 
muscles, the rectal sac is continued backwards for a variable distance 
as a pair of large diverticula, to which its bilobed appearance is due. 
The walls of the sac are excessively thin and unless the greatest care is 
taken, they are generally ruptured in making dissections of the entire 
animal, the opaque white granular contents—the secretion of the 
Malpighian tubules—being thereby scattered in the body cavity. Its 
structure most closely resembles that of the Malpighian tubules, though 
the epithelial lining is considerably reduced in thickness and has 
apparently no secretory function. The cytoplasm of the cells is so sparse 
that the nuclei actually exceed in their diameter the thickness of the 
