SECT. XVI PERIPATUS AND THE TRACHEATA 281 



The common derivation of the two divisions of the 

 Arthropoda from Annehds modified to use the para- 

 podia as jaws, &c., in the one case round a mouth 

 at the anterior end of the body, and in the other 

 round a mouth bent under so as to face posteriorly, 

 makes it possible, we think, for the first time 

 clearly to homologise the head regions of the two 

 divisions. 



The Annelidan prostomium became in both cases 

 the labrum. In both groups the Annelidan antennae 

 were retained as sensory organs, having disappeared 

 only in the Arachnida. The first pair of parapodia, 

 the antennal parapodia of the Annelids, became 

 differently modified on account of the different posi- 

 tion of the mouth. In the Crustacea the mouth was 

 carried round ventrally to between the parapodia of 

 the third and fourth segment?, which thus, in the 

 typical Crustacean head, became the chief jaws, leav- 

 ing the antennal parapodia as a rule free to continue 

 to function as .sensory organs. In the Tracheata, on 

 the other hand, the anterior position of the mouth 

 almost necessitated the formation of the chief jaws 

 out of the first pair of parapodia. In Peripatus these 

 alone function as jaws. In the Myriapoda and 

 Hexapoda they are the chief jaws, but are assisted by 

 the two following pairs as first and second maxillae. 

 In the Arachnida they form the powerful and vari- 

 ously modified chelicer3e which develop so largely as 

 to displace and lead to the degeneration of the pro- 

 stomium and antennae. These formidable jaws are 

 assisted by the second pair of parapodia as accessory 



