CCBLOM AND VASCULAR SPACE. 117 



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

 communicate with the vascular system j (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 euteron, or as a space or 

 spaces in the unsegmented or segmented mesoblastic bauds (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 MoUusca 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 coeloraic, 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 

 or less well-marked cavity are formed in the embryo; but the 

 fate of these structures has never been followed. We do not 

 know whether their cavities enlarge and unite with one another 

 and give rise to the body cavity and vascular system of the 

 adult, or whether they shrivel up and disappear, their walls 

 only remaining as part of the mesoderm. From what has 

 been said it is also clear that it is impossible to say whether in 

 the Arthropoda the vascular system is nipped off from the 

 coelom, or whether it arises as a separate set of spaces in the 

 mesoderm, as in Annelids and Vertebrates. 



I 



