THEORY OF EVOLUTION 483 



able til rough the larva. The fundamental morphology of the cteno- 

 phore is similar to that of the trochophore larva, and the Platy- 

 helminthes are generally thought to have been derived from Cteno- 

 phores. The regular arrangement of gonads and the even, orderly 

 distribution of the diverticula from the intestines of the flatworms 

 are interpreted by many to indicate preparation for segmentation 

 as it appears in annelids. 



Peripatus and the Wormlike Ancestry of Arthropoda 



Peripatiis, the only representative genus of class Onycophora, was 

 considered a segmented worm for a long time because of its shape, its 

 even, conspicuous segmentation, and possession of nephridia for ex- 

 cretion. But later it was discovered that the respiration is carried 

 on by tracheae, and the body cavity serves as a blood space, both 

 of which are typical arthropod features. In addition, the paired 

 legs are jointed, although similar in appearance to paropodia of 

 Annelida. There are two jointed antennae on the head and some 



Fig. 208. — Peripatus capensis. Natural size. (After Moseley from Folsom's 



EntovioLogy. Redrawn by Nelson A. Snow.) 



jawlike plates in the mouth. Situated as it is, midway between 

 annelids and arthropods, this form seems to show an immediate 

 transition from the one group to the other. Upon this basis it is 

 usually held that the arthropods have a wormlike ancestry. 



Interestingly enough, there exists another idea of arthropod an- 

 cestry from the fact that all of the lower forms of Crustacea pass 

 through a characteristic stage known as the Nauplius (Fig. 167). 

 This larva does not correspond very closely to any strictly annelid 

 stage, but with its short body and three pairs of appendages it re- 

 sembles a modified trochophore larva. The nauplius larva has some 

 features in common Avith the rotifers which authors feel may have 

 arthropod tendencies. 



Echinoderms arid Their Larval Relations 



Although the adult echinoderms possess radial symmetry, that 

 seems not to have been in the immediate phylogenetical background. 



