364 



EMBRYOLOGY 



the nephridia of the adult Sipiinculus are described as opening towards 

 the body cavity, and that the sexual glands are explained as growths of 

 the peritoneal epithelium, the products of which are set free in the body 

 cavity, and from there pass into the funnels of the nephridia. 



II.— The Development of Phascolosoma. 



According to Selenka's description, the development of 

 Phascolosoma elongatum is much simpler than that of tSipun- 

 culus. Following an unequal cleavage, there arises an 

 epibolic gastrula, which is said, however, to be converted into 

 a kind of invagination gastrula after the division and inva- 

 gination of the macromeres, which soon ensue. Cilia, which, 

 as in Sipunculus, perforate the egg-membrane, make their 

 appearance early. They form a tuft at the apical pole and 

 a post-oral ciliated band ; however, a preoral band is also 

 present, so that in Phascolosoma the resemblance to the 

 Tronhophore is greater. The blastopore is said to be directly 

 converted into the mouth ; the anus in this case, too, lies 

 on the dorsal side. The formation of an embryonal mem- 

 brane is not described by Selenka ; on the contrary, this in- 

 vestigator states that the egg-membrane 

 becomes the cuticula of the larva, as has 

 already been described for some Annelids. 

 The embryo (the larva) then simply elon- 

 gates, so that here a stage quite similar 

 to that in Sipunculus is reached, but in a 

 simpler manner. 



The larva, which, as one of the last 

 stages, was observed by Selenka, is elon- 

 gated (Fig. 160). The trunk, which is 

 much the more voluminous, is separated 

 from the small head portion by means of 

 a thick, collar-like ridge, which bears the 

 post-oral ciliated band. A large portion 

 of the head is occupied by the broad 

 preoral band of cilia, and at the anterior 

 end the ciliated tuft of the apical plate 

 projects. The head bears two eye-spots. The hooks which 

 constitute the armature of the adult animal make their ap- 



FiG. 160.— Larva of 

 Phascolosoma elonga- 

 tum (after Selenka). 

 b, setae ; m, mouth ; 

 w, preoral, w^, post- 

 oral, ciliated bands. 



