122 SUMMARY. 



animals, e.g. Annelida, Mollusca, the vascular space or 

 pseudoccele appears before the ccElom, in Peripatus the coelom 

 appears first, and that in Arthropods, at least, the vascular 

 space is in the early stages very coramonly occupied by yolk, 

 while the coelom is entirely free from yolk. This latter 

 fact would seem to imply some connection between the vas- 

 cular space and the enteric space; and I would also desire 

 to point out that the coelom, generative glands, and nephridia 

 can, in all animals whose development is at all well known, be 

 traced back to a very early embryonic structure, which appears 

 at the very beginning of development, gives rise to no other 

 structures, and itself arises in very diflFerent ways in different 

 animals. The embryonic structure I refer to is in some cases 

 the mesoblastic bands, and in others enteric diverticula. That 

 these two kinds of coelomic rudiments, as I may call them, are 

 homologous cannot be doubted, but which, if either, of the 

 methods of origin is primitive, cannot in my opinion at present 

 be settled. 



Summary of the Development of, and of the above 

 Remarks on, the Ccelom and Body Cavity. 



It is well known that the vascular system of the Arthropoda 

 is in direct communication with the body cavity, and that the 

 vessels are, for the most part, very rudimentary. In fact the 

 blood is driven by the heart or dorsal vessel into the body 

 cavity, and returned directly through the lateral cardiac ostia 

 into the heart. In no other group of animals does this direct 

 communication exist between the heart and the pericardium. 



It is therefore important to determine by the study of 

 development, whether or no the blood-containing body and 

 pericardial cavities of the Arthropoda are homologous with the 

 corresponding structures of other types, in which they do not 

 contain blood. 



The development of the Arthropodan heart and body cavity 

 is in most cases extremely diflficult to follow on account of the 



