590 



CHAETOGNATHA. 



No separate renal organs are known. 



Development. The ova are transparent structures which float on 

 the surface of the sea, except in Spadella cepJialoptera, which attaches 

 them to seaweeds. The mode of oviposition and the place of fertili- 

 zation is not known. The whole development takes place in the sea, 

 and there is no larval stage. The segmentation is complete, and leads 

 to the formation of a hollow blastophere, and an invaginate gastrula. 

 The blastopore closes at the posterior end of the embryo, and the 

 mouth is a new formation ft the anterior end. The archenteron 

 gives off two pouches from its posterior end (Fig. 471), and thus 

 becomes trilobed. The middle lobe acquires an opening to the exterior 

 at the front end, while posteriorly it opens into the part of the 



archenteron which 

 opens into the lateral 

 pouches (Fig. 471). 

 This hind opening 

 becomes closed, so 

 that the middle lobe 

 forms a tube opening 

 in front by the mouth, 

 and behind ending 

 blindly, and thus 

 gives rise to the en- 

 teron. The anus is 

 formed later. The 

 portion of the arch- 

 enteron thus sepa- 

 rated off is the coel- 

 om; it consists of two 



lateral sacs opening into an unpaired portion (Fig. 471) behind. 

 From the anterior end of the two lateral sacs a portion is cut off, 

 which apparently becomes the cephalic sections of the adult coelom 

 (Fig. 472). The further changes of the hinder part of the coelom 

 into the adult condition have not been followed. 



One of the most remarkable features of the development of Sagitta 

 is the early appearance of the generative cells, if indeed they be such. 

 These appear in the gastrula stage as a prominence of six cells 

 projecting from the hypoblast at the anterior end of the archenteron. 

 This mass, on the folding of the archenteric wall, is placed at the 

 hind end of the median lobe (Fig. 471), and when the division of the 

 archenteron takes place it remains in the coelomic portion (Fig. 472), 



FIG. 472. A, dorsal ; B, lateral views of an advanced embryo 

 of Sagitta (from Balfour, after Biitschli). m mouth; al 

 alimentary canal ; vg ventral ganglion ; ep ectoderm ; cpv 

 cephalic section of the body- cavity; so somatic; sp 

 splanchnic layer of mesoderm ; gc generative organs. 



