198 



OXYCHOPHORA. 



B. 



were originally related to the blastopore. v. KENNEL, however, does not 

 believe this, and, further, seems little inclined to attribute much value to the 

 observations on this point made in other species of Peripatus. He attributes 

 an altogether different significance to the groove in the blastoderm observed 

 by himself and described by us in accordance with the views of English authors 

 as the blastopore. 



The further development of the pharynx takes a course similar to that above 

 described, the primary oral aperture shifting inwards, while an anterior ento- 

 dermal diverticulum appears. The anal aperture, on the contrary, which arose 

 in front of the blastopore through the formation of a slit (Fig. 89 A), is said not 

 to coincide with the adult anus. The former closes by the approximation 

 of its edges, and an ectodermal invagination arises a little distance in front of 

 it, grows inward towards the entoderm and fuses with it. The rectum and 

 anus are thus formed, the latter then shifting more to the posterior end of the 

 embryo in consequence of the unequal growth of the latter (v. Kennel). 



The Mesodermal Structures. 



The formation of the 

 chief mass of the meso- 

 derm proceeds from a 

 zone of growth lying at 

 the posterior end of the 

 blastopore, and extends 

 forward from this point 

 in the form of two bands 

 (mesoderm - bands) lying 

 symmetrically to the ven- 

 tral median line. Where 

 a slit -like blastopore is 

 present, as in the African 

 and Australian species, 

 the mesoderm- bands lie 

 in close contact with it, 

 and are thus situated in 

 the region where the 

 ectoderm passes into the 

 entoderm. After the 

 blastopore has partially 

 closed, the posterior (anal) 

 aperture lies in front of 

 the growing zone, and its 

 position is the same in 



Fig. 99. — Ventral aspect of embryos of P. capcnsis, to . , . . . 



illustrate the segmentation of the mesoderm (after the American species, 111 



Balfour and Sedgwick), a, anus ; bl, blastopore ; which the blastopore is 



m, mouth; »s, primitive segments; w, zone of ... 



growth, not slit-like. 





