118 GENERAL CONSIDERATIONS. 



NoWj Peripatus is a true Arthropod so far as its body 

 cavity is concerned : thus the heart drives the blood into 

 it, and by means of the paired cardiac ostia sucks the blood 

 out of it ; it does not communicate with the exterior by nephri- 

 dial pores, nor does its lining develope generative cells. We 

 are therefore justified in regarding the body cavity of Peripatus 

 as homologous with that of other Arthropoda. It results 

 from this that the study of the development of the body 

 cavity in Peripatus, which can be traced with comparative 

 ease, must be of extreme interest, as tending to clear up the 

 question of its ccelomic or non-ccelomic nature in Arthropoda 

 generally. 



Kennel was the first to trace the body cavity of Peripatus. 

 He showed that it was in part, at any rate, a pseudocoele, but 

 his work was incomplete in that he failed to follow correctly 

 the fate of the coelom. He thought that the coelom became 

 merged into the body cavity. If this were correct, it would 

 follow that in Peripatus the vascular system and coelom would 

 be in communication. 



As has been fully shown in the preceding pages, this is not 

 the case. The coelom of Peripatus can be traced through the 

 whole development, as a system of spaces shut ofi" at all stages 

 of its growth from the system of body-cavity spaces. In the 

 adult Peripatus the coelom is in the following condition : (1) a 

 series of nephridia ending internally in small thin-walled 

 closed vesicles ; (2) two dorsal tubes — the generative glands 

 and the ducts of these, which latter are derived from one pair 

 of posterior somites. The pericardium, heart, whole of the 

 body cavity (central, lateral, and leg compartments) are exclu- 

 sively pseudocoelic in origin. 



In Peripatus, therefore, the gonads are coelomic, and their 

 ducts what Lankester would call nephrodinic. 



The condition of the body cavity and coelom of Peripatus 

 will be best appreciated by comparing it with that of the same 

 organs in an Annelid, such as Lumbricus. 1. In Lumbricus 

 the structures corresponding to the nephridial vesicles of 

 Peripatus have swollen up and united with one another in pairs 



