NO. II LEGS OF PRIMITIVE ARTHROPODS EWING 25 



axis and each bearing a pair of similar segmental appendages. Evolu- 

 tionary forces have not only tended to bring about diversity among 

 these different segments and their apjiendages but have caused cer- 

 tain segments either to fuse or to act together more or less as a unit 

 in performing certain classes of functions. Thus in certain arthropods 

 a group of the most anterior segments became fused to form the head, 

 in which most of the special senses were lodged, others posterior to 

 the head in certain arthropods became grouped or fused together to 

 form the thorax which became chiefly devoted to locomotion, and 

 lastly the remaining segments became an abdomen, a unit having to 

 do largely with digestion and reproduction. 



In the insects alone do we find these three body regions completely 

 developed and students of the evolution of this class are particularly 

 interested in finding out how many and what segments of the prim- 

 itive arthropod enter into the formation of the insect head, thorax 

 and abdomen. 



That the head was the first of these three body regions to become 

 differentiated appears evident, for in a number of classes of land 

 arthropods it is completely formed while the remainder of the body 

 is composed of many segments, the most of which are very similar and 

 have each a pair of similar appendages. 



In discussing the origin of the thorax we are particularly inter- 

 ested in the origin of certain structures found in the insect neck re- 

 gion. If the so-called cervical sclerites are true sclerites, could they 

 have been derived from the basal segments of an arthropod appen- 

 dage, and if so does the neck itself represent the rudiments of a 

 postcephalic segment? 



THE FIRST BODY SEGMENT OF .SYMPHYLA 



In the Symphyla it is observed that the first pair of legs are always 

 reduced. In some instances this is due only to the shortening of cer- 

 tain segments, particularly the femur. In Hanseniella, however, one 

 segment, the femur, has dropped out entirely. In other symphylids 

 the leg becomes quite rudimentary. 



THE FIRST BODY SEGMENT IN PAUROPODA 



The evidence of a much reduced first body segment with vestigial 

 legs is very convincing in the case of pauroixjds. Not only is there 

 a complete suture setting oft' a neck region but on the sternum there 

 is a pair of very short disclike structures that should be regarded as 

 leg rudiments. Kenyon (1895) pointed out that these structures each 



