GEPltrEEA. CH.ETIFEHA. 



391 



posterior segmental organs (anal vesicles, tig. 314, Ab) in the 

 terminal segment, eacli of which has a number of peritoneal 

 funnels and opens into the rectum. In Bonellia the segmental 

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

 single (fig. 314 b). 



Development. The development of the ovum begins with an 

 unequal segmentation. In Bonellia the small ceils 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 

 Echiurus larvae (fig. 315) are the most accurately known. They 

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



a 



s? 



A 



FIG. 315. a, Larva of Eclriurus from flic ventral side (after Hatsclaek). SP, apical plate; 

 Prw, prseoral circle of cilia ; Pote, postoral circle of cilia; ~ii, head-kidney; Vg, ventral 

 Kanglionic cord connected with the apical plate by the long ossophageal commissures ; 

 AS, anal vesicle, b, Ventral region of the Echiurus larva with segmented mesodermal 

 bands; SC, oesophageal commissure ; Dsp, dissepiments of the anterior body segments j 

 MS, mesodermal bands ; A, anus. 



prrcoral 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 pronephros (KN), is developed, 

 one on either side ; and behind it a pair of nie.soblastic 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 segmental 



