542 SUMMARY. 



doubt that the Tracheata are descended from a terrestrial Anne- 

 lidan type related to Peripatus. The affinities of Peripatus to 

 the Tracheata are, as pointed out in a previous chapter (p. 386), 

 very clear, while at the same time it is not possible to regard 

 Peripatus simply as a degraded Tracheate, owing to the fact 

 that it is provided with such distinctly Annelidan organs as 

 nephridia, and that its geographical distribution shews it to be a 

 very ancient form. 



The Crustacea on the other hand are clearly descended from 

 a Phyllopod-like ancestor, which can be in no way related to 

 Peripatus. 



The somewhat unexpected conclusion that the Arthropoda 

 have a double phylum is on the whole borne out by the anatomy 

 of the two groups. Without attempting to prove this in detail, 

 it may be pointed out that the Crustacean appendages are 

 typically biramous, while those of the Tracheata are never at 

 any stage of development biramous 1 ; and the similarity between 

 the appendages of some of the higher Crustacea and those of 

 many Tracheata is an adaptive one, and could in no case be 

 used as an argument for the affinity of the two groups. 



The similarity of many organs is to be explained by both 

 groups being descendants of Annelidan ancestors. The simi- 

 larity of the compound eye in the two groups cannot however 

 be explained in this way, and is one of the greatest difficulties 

 of the above view. It is moreover remarkable that the eye of 

 Peripatus 2 is formed on a different type to either the single or 

 compound eyes of most Arthropoda. 



The conclusion that the Crustacea and Tracheata belong to 

 two distinct phyla is confirmed by a consideration of their 

 development. They have no doubt in common a centrolecithal 

 segmentation, but, as already insisted on, the segmentation is 

 no safe guide to the affinities. 



In the Tracheata the archenteron is never, so far as we 

 know, formed by an invagination 3 , while in Crustacea the 



1 The biflagellate antennae of Pauropus amongst the Myriapocls can hardly be 

 considered as constituting an exception to this rule. 



3 I hope to shew this in a paper I am preparing on the anatomy of Peripatus. 



8 Stecker's description of an invagination in the Chilognatha cannot be accepted 

 without further confirmation ; -vide p. 388. 



