540 



INVERTEBEATA 



CHAP. 



of the mother, and here they are fertilized. They develop veiy 

 rapidly ; the segmentation of the egg and formation of the layers is 

 exactly like what it is in Synapta digitata. The embryo, however, 

 never develops either the folded ciliated band of the Auricularia or 

 the transverse bands of the pupa ; it is an oval organism uniformly 

 ciliated all over. The stomodaeum arises on the ventral side, and 

 becomes displaced to the left and then to the anterior pole. The 

 larval anus disappears early, and the anus is re-formed later in 

 development. The genital organ arises as a thickening of the 



Fro. 401. — Pupa of Synapta digitata in two stages of development. (After MetschnikofF. ) 



A, before tlie buccal tentacles have been protruded from the atrinm ; note the five transverse ciliated 

 bands. B, after the buccal tentacles have been protruded from the atrium; note the four transverse 

 ciliated bands. Letters as in previous figure and in Fig. 398. In addition, at, atrium, i.e. larval 

 stomodaeum ; calc.h, rudiment of buccal skeleton ; ot, rudiment of otocyst ; Pol, rudiment of Polian 

 vesicle ; st.c, stone-canal. 



epithelium covering the right hand side of the mesentery carrying 

 the stone-canal, i.e. from where the right horn of the left posterior 

 coelom ivipinges on the anterior coelom; the enlarged cells burrow 

 into the gelatinous substances of the mesentery, there form a 

 spherical mass which grows out into the genital tubes, and later 

 forms its own duct leading to the exterior. 



Beyond all doubt the process of the development of the genital 

 organs is the same in Synapta digitata. Stages in the development 

 of the genital organs of Gucuynaria glacialis have been observed by 

 Mortensen, who confirms Clark's account (190-1:). The spot where 

 the genital cells make their appearance is therefore the same in 



