92 THE CHANGES FROM STAGE A TO P. 



legSj 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 p an important change takes place : the 

 pseudocoele or body cavity of the leg makes its appearance. 

 It arises simply as a space in the mass of mesoderm which 

 occupies the periphery of the appendage (Plate IX, figs. 

 53, 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, aud 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, l.s.v.S, Plate "VIII, and description above, p. 79). 



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. 



The tubular part, which in the case of the first three legs 

 remains straight even in the adult, becomes the structure 

 which was first described by Balfour (excepting (?) Saenger, 

 whose paper I cannot procure) and called by him the nephri- 

 dium ; while the internal vesicular part, which persists through- 



