DEVELOPMENT OF THE CAPE SPECIES OF PERIPATTJS. 533 



the fifth or sixth somite — not merely the mesoderm, but all 

 the layers — derives its nuclei from the primitive streak. The 

 primitive streak nuelei are therefore not merely mesodermal, 

 but ectodermal and endodermal as well. 



3. The last feature in the development of Peripatus 

 capensis which I would desire to notice in its general bear- 

 ings, is the development of the body cavity and the fate of 

 the coelom. 



The ccelom, as is well known, is the term applied to a body 

 cavity with certain characters — characters which may be 

 summed up in the following terms : — (1) The coslom does not 

 communicate with the vascular system ; (2) it communicates 

 with the exterior by nephridial pores ; (3) its lining gives rise 

 to the generative products ; (4) it develops either as one or 

 more diverticula from the primitive enteron, or as a space or 

 spaces in the unsegmented or segmented mesoblastic bands (in 

 the latter case called mesoblastic somites). 



The vascular space has none of these characters, and is 

 known as a pseudocoelic space: it develops either from the 

 blastocoele or from a system of channels hollowed out in the 

 mesodermic tissue of the body. In the Annelida and Ver- 

 tebrata these two spaces co-exist, and present a well-marked 

 contrast to one another ; while in the two other great groups 

 of the animal kingdom — the Mollusca and Arthropoda — 

 the relations of the two systems has not been thoroughly 

 understood. We will first consider the case of the Arthro- 

 poda. 



The body cavity in the Arthropoda has generally been 

 regarded as coelomic, in spite of the fact that it presents none 

 of the ordinary coelomic characters. It communicates with the 

 vascular system, it does not open to the exterior by nephridial 

 pores, its lining does not, so far as is known, develope the 

 generative cells, for the generative glands are continuous with 

 their ducts, and, so far as is known, have no connection with the 

 body cavity. Neither has the body cavity been traced into 

 connection with the undoubted coelom of the embryo. In all 

 the groups of the Arthropoda mesoblastic somites with a more 



