362 



EMBRYOLOGY 



7^, 



the adult worm. This applies, for example, to the arrange- 

 ment of the mesoderm, which is seen clothing the walls of 

 the body and intestine as the somatic and splanchnic layers, 

 though, according' to Hatschek, the somatic layer also sup- 

 plies the covering of the fore- and hind-guts, so that only 

 the covering of the entodermal part of the intestine (the 

 mid-gut) arises from the splanchnic layer. From the somatic 

 layer arise also the four retractors of the anterior part of 

 the body, which are developed even in the larva, and extend 

 from the head region to the anus (Fig. 159 r). In conse- 

 quence of this arrange- 

 ment, the anterior part 

 of the larva can be in- 

 vaginated into the pos- 

 terior part. A circular 

 muscle lying behind the 

 ciliated band (Fig. 159 

 rm) serves to close the 

 opening of the invagina- 

 tion in the larva, which 

 in this retracted condi- 

 tion is almost spherical. 

 The paired nephridia, 

 which in their structure 

 correspond to those of 

 the Annelida, are also 

 produced from the soma- 

 tic layer of the meso- 

 derm. At quite an early 

 stage of the embryo a 

 mesoderm cell was dis- 

 tinguishable from the 

 rest by its strikingly- 

 yellow colour. Some 

 other cells were then 

 added to it. The entire 

 structure assumed a looped form, and a lumen was excavated 

 inside it (Figs. 158 F, 159 n). There are cells, likewise of 

 mesodermic origin, resembling blood corpuscles, which were 



Fig. 159.— Larva of Sipunculus nudus (after 

 HiTscHKK). a, anus; 61, cells resembling 

 blood corpuscles; dr, glandular appendage 

 of the fore-gut ; in, mouth ; n, nephridium ; r, 

 retractors; nti, circular muscle ; s, pharynx ; 

 ap, apical plate. 



