MUSEUM OF COMPARATIVE ZOOLOGY. 85 



composed of tKree layers : an internal epithelial layer, composed of the 

 flattened ectodermic cells with oval nuclei ; a middle layer, composed of 

 very much flattened cells, recognizable only by their long very narrow 

 nuclei ; and an external covering of mesodermic elements (PI. IX. 

 fig. 57). 



A few days before hatching a short tube is to be seen extending for- 

 wards from the ventral face of the stercoral pocket to which it is joined. 

 It is continuous behind with the part of the proctodaeum which I have 

 called the rectum, and in front it spreads out into a trumpet-shaped 

 expansion which embraces the posterior portion of the yolk mass 

 (PI. VIII. figs. 55, 56 ; PI. XI. fig. 70). The cells composing the epi- 

 thelial lining of this portion of the intestine resemble more those of the 

 stercoral pocket than those of the rectum. They are invested externally 

 by a layer of flattened mesodermic elements continuous behind with those 

 which envelop the stercoral pocket and the rectum, and in front with 

 the mesodermic layer which invests the yolk. 



Schimkewitsch claims for the hind part of the alimentary canal in 

 Epeira a very thin cuticular lining ; I have not been able to demonstrate 

 its presence in sections of Agelena. 



From the dorsal wall of the pre-stercoral tube — just where it becomes 

 confluent with the antero-inferior face of the stercoral pocket — the two 

 malpighian tubes take their origin. The position of these tubes furnishes 

 the only evidence that I have concerning the source of the pre-stercoral 

 tube, and leads to the conclusion that it is of entodermic origin, and 

 therefore a part of the mesenteron. 



I have not as yet traced the alimentary canal to its adult condition. 

 At the time of hatching it is still incomplete, being composed of an 

 anterior and a posterior portion, the inner extremities of which open 

 towards each other by wide expansions, which abut directly upon the 

 yolk. In the latest developmental condition that I have examined ■•— 

 about eight or ten days after hatching — the mesenteron is greatly ex- 

 tended, and appears to be continuous at its sides with the yolk compart- 

 ments of the abdomen. It doubtless is functionally active previous to 

 this time, since there was a considerable amount of effete matter in the 

 stercoral pocket. 



2. The Eyes. — It seems somewhat remarkable that up to the present 

 time the development of the sense organs in the Araneina has been 

 hardly more than touched upon. The meagre description by Claparede * 



* Claparede ('62, pp. 56, 67) is the only one, T believe, who has written anything 

 about the development of the eyes, and he has given only an account of the exter- 



