3o8 



ZOOLOGY 



of two, if not three, of the most important phyla of the animal 

 kingdom. Its develop ement is no less interesting; it would 

 be beyond the scope of this book to dwell upon the early 

 stages of the embryology, but some of the later stages 

 have cleared up so many difficulties in the structure of 

 adult Arthropods that a short reference to them must be 

 made. 



The morphological nature of the body-cavity, and of the 

 heart, which opens freely into it in all Arthropods, has always 



Fig. 176. — Difigram of the arrangement of the ooelomic and haemoooelio cavities of 

 Peripatus, at the time of its birth. After Sedgwick. 



1. Enteron, alimentary canal. 3'. Haemoooelio cavity of heart. 



2. Coelomic cavity of generative gland. 4. Nervous system. 

 2'. Coelomic cavity of nephridia. 5. Slime glands. 



3. Haemocoelic cavity, general "body- 



cavity." 



The true coelom is everywhere surrounded by a thick black line. 



been a difficulty ; the developement of Peripatus shows us that, 

 in this genus at all events, it has no connection with the true 

 coelom, but is a haemocoel, and arises partly by a hollowing 

 out in the tissue of the mesoblastic somites and partly from 

 the persistence of a split between the ectoderm' and endoderm. 

 The continuity of the walls of the generative organs with the 

 genital ducts is explained by the fact that the generative 



