212 ONYCHOPHORA 



features in common with these two divisions, some of these occurring 

 in its ontogeny. 



Although the eggs of some species of Peripatus have little, or even 

 no yolk, it is highly probable that they are to be traced back to eggs 

 rich in yolk, like those of P. novae-zealandiae* These undergo 

 superficial cleavage and become covered with a blastoderm, and thus 

 resemble the eggs of the majority of Arthropods. The long, slit-like 

 blastopore which, in closing, leaves the oral and anal apertures, finds 

 its homologue among the Insecta. The formation of the germ-layers 

 can also be compared with processes found among the Insecta or the 

 Crustacea, but in the development of the mesoderm -bands from 

 the edge of the blastopore and their gradual shifting forwards, and 

 in the nature of the segmentation of these bands, we find an agree- 

 ment with the Annelida. The same is true with regard to the 

 form of the germ -bands, which is, of course, largely determined 

 by the nature of the mesoderm-bands. Balfour pointed out the 

 great similarity between the germ-bands of Peripatus and those of 

 the Myriopoda and the Arachnida (e.g., Geoplrilus, Scorpio, Agalena), 

 which is shown in the form of the limb-rudiments, and especially 

 in "that of the cephalic lobes. But we must remember that the 

 formation of such germ-bands was first introduced in the Annelida 

 ( Oligochaeta, Hirudinea). 



The great development of the brain and the possession of limbs 

 produces again a greater resemblance to the Arthropoda. This 

 resemblance finds further expression in the union of several segments 

 to form the head, and in the transformation of their appendages 

 (limbs) into mouth-parts. The inclusion of one or more trunk- 

 segments in the head has, indeed, been stated for the Annelida, but 

 these segments are never so radically transformed as in the Arthro- 

 poda and in Peripatus. Such agreement suggests the cpuestion 

 whether the cephalic segments of the Onychophora and those of 

 the Arthropoda may not be homologous, but we are here met 

 with difficulties, the number of the segments involved in the 

 formation of the head differing in the two groups, and the relation 

 of the segments to each other also varying. The latter finds its 

 expression in the composition of the brain. In Peripatus the 

 ganglia of the maxillary segment are included in the formation 

 of the brain, which is not the case in the Myriopoda and Insecta. 

 The jaws of Peripatus cannot, therefore, be homologised with the 



* [For Willey's conclusions (App. to Lit. on Onychophora, Xo. II.) see 

 footnotes, pp. 165 and 216. — Ed.] 



