200 



ONYCHOPHORA. 



as. 



they show the ectoderm with its ventral thickenings, and the two 

 niesoderm-segments containing the primitive body-cavity, bounded 

 by the epithelial walls, applied to the ectoderm and the entoderm 

 (Fig. 100). 



Such anatomical and histological differentiation is present in the 

 embryo represented in Fig. 88, and no further essential change 

 appears until twelve to fifteen segments are visible externally, 

 together with the full (adult) number of internal segments 

 (v. Kennel). 



When the mesoderm-bands have broken up into the series of 

 consecutive primitive segments, the resemblance with the Annelida 

 is very striking, but the further course of development differs, 

 inasmuch as it is not the segmental cavities which yield the 



A. 



1. 



nus. 



us. 



vor.tn 



Fig. 100. — Transverse sections through embryos of P. capensis (A) and P. Edwardsii (P.) (after 

 Sedgwick and v. Kennel). A, transverse section through the region of the oral papillae in 

 an embryo at about the stage depicted in Fig. 91 A. B, transverse section through a trunk- 

 segment of a young embryo, d, intestine (entoderm) ; Ih, dorsal and ventral spaces between 

 ectoderm and entoderm (parts of the primary and adult body-cavity) ; I, lateral, m, 

 median portions of the segmental cavities ; mes, portions of mesoderm detached from the 

 primitive segments; n, rudiment of the ventral cord; us, primitive segment; vo, ventral 

 organ ; vo+n, common thickening of the ventral organ and the ventral nerve-cord. 



body-cavity of the adult, for, in Pervpatus, the latter arises as a 

 pseudocoele independent of the primitive segments. All that is 

 retained of these segments enters into the formation of the nephridia 

 and the genital organs (v. Kennel, Sedgwick). 



The formation of the future body-cavitj' and of the nephridia is 

 commenced by a thickening of the ventral Avail of the primitive 

 segments ; and subsequently, by an ingrowth of the cells of this 

 thickening, a separation of the segmental cavity into two spaces is 

 brought about, one dorso-median and the other lateral (Fig. 100 B, 

 m and 1) ; these are at first connected, but become completely 

 separated later (Fig. 104 A, p. 210). The dorsal portion shifts 



