GEPHTRBA. CH.ETIFEEA. 



391 



posterior segmental organs (anal vesicles, fig. -'314, Ab) in the 

 terminal segment, each of which has a immber of peritoneal 

 funnels and opens into the rectum. In Bonellia the segmental 

 organ which performs the function of uterus is, like the ovary, 

 single (tig. 314 ft). 



Development. The development of the ovum begins with an 

 unequal segmentation. In Bonellidi the small cells of the animal 

 pole grow round the four large yolk spheres, which give rise to the 

 entoderm, leaving a small aperture, the blastopore (fig. 110). The 

 Ecldnrus larva- (iig. 315) are the most accurately known. They 

 present the type of Loven's larva and possess a strongly developed 



FIG. 315. n, Larva of Echiui-us from the ventral side (after Hatschek). SP, apical plate; 

 Prw, prreoral circle of cilia ; Pow, postoral circle of cilia; En, head-kidney ; Vg, ventral 

 ganglionic cord connected with the apical plate by the long- cesophageal commissures ; 

 AS, anal vesicle. l>, Ventral region of the Echlurus larva with segmented mesodermal 

 hands ; SC, oesophageal commissure ; Dap, dissepiments of the anterior body segments ; 

 MS, mesodermal bands ; A, anus. 



prseoral circle of cilia (Prw), in addition to which there is also a 

 delicate post-oral circle of cilia (Pow). Early in larval life a seg- 

 mental organ, the head kidney or pi'onephros (A'-V r ), is developed, 

 one on either side; and behind' it a pair of mesoblastic bands makes 

 its appearance and gives rise in the subsequent development to the 

 rudiments of 15 segments (fig. 315 b). In the terminal segment, 

 which is surrounded by a circle of cilia, there appear segmeutal 



