SUMMARY. 451 



Summary of Arthropodan Development. 



The numerous characters common to the whole of the Arthropoda 

 led naturalists to unite them in a common phylum, but the later 

 researches on the genealogy of the Tracheata and Crustacea tend to 

 throw doubts on this conclusion, while there is not as yet sufficient 

 evidence to assign with certainty a definite position in either of these 

 classes to the smaller groups described in the present chapter. There 

 seems to be but little doubt that the Tracheata are descended from 

 a terrestrial Anuelidan type related to Peripatus. The affinities of 

 Peripatus to the Tracheata are, as pointed out in a previous chapter 

 (p. 319), 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 Anuelidan 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 similarity 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 evidence is in 

 favour of such an invagination being the usual, and, without doubt, 

 the primitive, mode of origin. 



1 The biflagellate antennae of Pauropus amongst the Myriapods can hardly be con- 

 sidered as constituting an exception to this rule. 



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



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

 without further confirmation; vide p 321. 



292 



