308 ZOOLOGY 
of two, if not three, of the most important phyla of the animal 
kingdom. Its developement 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 
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Frc. 176.—Diagram of the arrangement of the coelomic and haemocoelic cavities of 
Peripatus, at the time of its birth. After Sedgwick. 
1. Enteron, alimentary canal. 
31, Haemocoelic cavity of heart. 
) 
Coelomic cavity of generative gland. 4. Nervous system. 
21, 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 
