1918.] NATURAL SCIENCES OF PHILADELPHIA. 83 



by Tillyard (Proceedings of the Linnean Society of New South 

 Wales, 1915, Vol. XL, Part 3, August 25), have a much greater 

 folding of the rectal epithelium and no caudal gills. To quote him: 

 The rectum presents "six longitudinal double rows of lamellate 

 folds." It is within the thicker, unpigmented portion of the rectal 

 epithelium of A. talamanca that the fine ends of the tracheoles 

 which reach the epithelium are embedded. Though the number 

 of these embedded tracheoles is small, numerous tracheae were 

 traced into the folds and almost to the epithelium. This would 

 suggest that diffusion of oxygen takes place more easily through 

 the unpigmented portions of the folds. The secondary folds increase 

 the diffusion area. The fact that no tracheoles were traced through 

 or even up to the pigmented epithelium between the folds suggests 

 that these areas are reserved for the osmosis of carbonic acid gas 

 from the body tissues into the rectum. For says Tillyard, "It 

 is well known . . . that chitin is a colloid substance which 

 admits of the passage through it of gases by cUffusion, and is partic- 

 ularly- partial to carbonic acid gas. " 



The absence of folding of the epithelium at the extreme end of 

 the rectum probably merely facilitates the quick and constant 

 flow of water. 



The rectal trachece.— The tracheae that supply the rectum of A. 

 talamanca have been studied from a series of transverse sections 

 prepared by and in the possession of Dr. Calvert. Owing to the 

 decay of the anterior rectal epithelium some smaller tracheoles 

 may have been lost. 



Two main dorsal longitudinal tracheae have been traced, one 

 on the left (Idt of figs. 1 and 2), the other on the right {i^dt). 



These divide at the posterior end of the ninth segment respectively 

 into the left caudal gill trachea (Icgt) and left median caudal gill 

 trachea (Imcgt) and into the right caudal gill trachea (rcgt) and the 

 right median caudal gill trachea (rmcgt). 



Within the posterior half of segment VIII arise from the longi- 

 tudinal dorsal tracheae, a right dorsal rectal trachea (rdrti) and a 

 left dorsal rectal trachea (Idrti), both of which pass mesad and caudad 

 into the dorsal longitudinal rectal fold where they divide and 

 subdivide. Some of the smaller tracheoles enter the epithelium, 

 but no anastomoses were observed. 



From the lateral tracheae {rlt and lit), which are connected with 

 the dorsal longitudinal trunks in the middle of segment VIII, there 

 come off ventrally in the anterior half of segment VIII, the lateral 



