282 THE APODIDJE PART n 



jaws, supplied with long feelers, or as powerful chelate 

 limbs. 



The second pair of parapodia, which in the typical 

 Crustacean head become the chief mandibles, form, in 

 Peripatus, the oral papillae ; the slime glands opening 

 at their tips being perhaps homologous with the 

 acicular glands of the Annelidan parapodia. In the 

 Myriapoda and Hexapoda they become the anterior 

 maxillae ; and in the Arachnida they form the pedi- 

 palps or their homologues. 



The third pair of parapodia, which in the Crustacea 

 form typically the first pair of maxillae, in Peripatus 

 and the Arachnida function as the first pair of feet. 

 In the Myriapoda and Hexapoda they form the 

 posterior maxillae. 



Just as we saw that all the Crustacean groups, how- 

 ever aberrant, must have been derived from the same 

 bent Annelid, so we would deduce all the groups of 

 the Tracheata from the same Tracheatan-Annelid. 

 We find the same variety in the arrangement and 

 form of jaws, limbs, &c., and the same variety in the 

 number of segments. In both cases some of the 

 groups can be shown to have been differentiated direct 

 from the original Annelid, while others are only later 

 modifications of such groups. In the Crustacea we 

 think the Apodidae, and the Trilobites, were original 

 differentiations ; in the Tracheata, the Arachnida, the 

 Protracheata, and the Myriapoda. 



Turning now to the important morphological 

 characteristics common to all the Tracheata, viz. 

 the tracheae and the Malpighian tubules, we shall 



