230 



INVERTEBKATA 



CHAP. 



The development of the genital organs of the spider bears a striking 

 resemblance to their development in Astacus. 



In the caudal lobe the first trace of the mid-gut now appears. 

 An accumulation of yolk cells in the shape of a plate is formed. This 

 plate becomes bent into a U-shape so as to enclose a cavity which is 

 the rudiment of the stercoral pocket of the spider. Towards the 

 main mass of yolk the stercoral pocket is closed by an accumulation 

 of yolk cells. In this accumulation a cavity appears which develops 

 into the hinder portion of the mid-gut, and which is, so to speak, a 

 forward extension of the stercoral pocket. As it grows in length 

 the stercoral pocket is pushed backwards. From the pocket itself 

 two lateral outgrowths are given off which form the so-called 

 Malpighian tubes, the excretory organs which the spider possesses 



map 



'nap 



FIG. 179. Two sections through the developing stercoral pocket and Malpighian tubes 

 of Agelena labyrinthica. (After Kautsch.) 



A, the posterior section. B, the anterior section, map, rudiment of Malpighian tube ; 

 ster, rudiment of stercoral pocket. 



in addition to the coxal gland (Figs. 179 and 180). The ectoderm of 

 the ventral face of the caudal lobe is thickened so as to form the 

 so-called anal ganglia, which arise at the posterior point of union 

 of the two longitudinal rudiments of the nervous system. Just in 

 front of this point the proctotiaeum appears as an ectodermal in- 

 vagination, and this invagination soon afterwards will open into the 

 stercoral pocket. 



The persisting abdominal appendages now undergo further 

 changes. On the posterior aspect of the first two pairs, near their 

 bases, an ectodermic invagination is formed. In the case of the first 

 pair this invagination forms the lung sac. This appendage develops 

 on its upper and posterior face outstanding folds which are the 

 rudiments of the first lung-lamellae. The other lamellae of the lung 

 are formed by outgrowths from the thickened ventral wall of the 

 lung sac or to put it in another way, from the basal portion of the 

 appendage. The credit of having given the first clear account of 



