550 E. RAY LANKESTEE. 



bemlgnathous mandibles. The Crustacea are tetartogna- 

 tlious. 



The history of the development of the head has been cai*e- 

 fully worked out in the Hexapod insects. As in Crustacea 

 and Arachnida, a first prosthomere is indicated by the paired 

 eyes and the protocerebrum ; the second prosthomere has a 

 well-marked coelomic cavity, carries the antennte, and has the 

 deuterocerebrum for its neuromere. The third prosthomere 

 is represented by a well-marked pair of coelomic cavities and 

 the tritocerebrum (III, fig. 6), but has no appendages. They 

 appear to have aborted. The existence of this third prostho- 

 mere, corresponding to the third prosthomere of the Crustacea, 

 is a strong argument for the derivation of the Hexapoda, and 

 with them the Chilopoda, from some offshoot of the Crustacean 

 stem or class. The buccal somite, with its mandibles, is in 

 Hexapoda, as in Crustacea, the fourth : they are tetarto- 

 gnathous. 



The adhesion of a greater or less number of somites to the 

 buccal somite posteriorly (opisthomeres) is a matter of im- 

 portance, but of minor importance, in the theory and history 

 of the Arthropod head. In Peripatus no such adhesion or 

 fusion occurs. In Diplopoda two opisthomeres — that is to 

 say, one in addition to the buccal somite — are united by a 

 fusion of their terga with the terga of the prosthomeres. 

 Their appendages are respectively the mandibles and the 

 gnathochilarium. 



In Arachnida the highest forms exhibit a fusion of the 

 tergites of five post-oral somites to form one continuous cara- 

 pace united with the terga of the two prosthomeres. The 

 five pairs of appendages of the post-oral somites of tlie head 

 or prosoma thus constituted all primitively carry gnathobasic 

 projections on their coxal joints, which act as hemignaths; in 

 the more specialised forms the mandibular gnathobases cease 

 to develop. 



In Crustacea the fourth or mandibular somite never has 

 less than the two following somites associated with it by the 

 adaptation of their appendages as jaws, and the ankylosis of 



