DEVELOPMENT OF THE CAPE SPECIES OF PERIPATUS. 509 



of the anterior somites are small spaces in the base of the legs 

 with a ventral prolongation which lies along the outer edge of, 

 and, except in the case of legs 4 and 5, opens to the exterior 

 immediately outside the nerve-cord. In the case of these 

 legs, the opening is a little removed from the nerve-cord and 

 placed on the ventral surface of the leg itself. 



At the end of Stage f an important change takes place : the 

 pseudocoele or body cavity of the leg makes its appearance. 

 It arises simply as a space, which is from the first somewhat 

 irregular and traversed by cells, in the mass of mesoderm 

 which occupies the periphery of the appendage (PI. XXXVII, 

 figs. 52, 53 a, h, h.app). The space almost at once becomes 

 much larger and more conspicuous than the lateral compart- 

 ment of the coelom, the outer wall of which — i. e. the wall 

 which separates it from the new cavity — is extremely thin and 

 delicate. So thin and delicate indeed, and so sudden the 

 appearance of the leg pseudocoele, that I was for a long time 

 inclined to the opinion that the latter was derived from a 

 part of the lateral compartment of the coelom. I have, how- 

 ever, convinced myself, by prolonged and careful study of my 

 sections, that this is not the case, but that the pseudocoele of 

 the leg, both in its origin and subsequent history, has nothing 

 to do with, and is entirely separate from, the lateral or 

 nephridial compartment of the coelom. The extreme tenuity 

 of the outer wall of the lateral coelom of the legs is shared by 

 the same structure in the oral papillae (third somite) [vide figs. 

 38, 38 a, /.«.«;. 3, PI. XXXVI and description above, p. 449). 



By the close of Stage f we can distinguish, as in the case 

 of somite 3, two parts in the lateral compartments of the 

 somites, viz. (1) an internal vesicular part, with an internal 

 and dorsal wall in which the nuclei are far apart and separated 

 by a relatively large amount of little staining protoplasm, and 

 an external wall of considerable tenuity separating it from the 

 secondarily developed body cavity of the leg ; and (2) a tubular 

 part which leads ventrally to the external opening; and even 

 in this stage, except in the case of the first three legs (somites 

 IV — vi), has begun to become convoluted. 



