STRUCTURE AND CLASSIFICATION OF THE AETHROFODA. 545 



as well as of one or more somites behind it (opisthouieres). 

 The first of the post-oral somites invariably has its parapodia 

 modified so as to form a pair of hemignaths (mandibles). 

 Twenty-five years ago the question arose as to whether the 

 somites in front of the mouth are to be considered as derived 

 from the prostomium of a Ch^topod-like ancestor. Milne- 

 Edwards and Huxley had satisfied themselves with discussing 

 and establishing*, according to the data at their conimand^the 

 number of somites in the Arthropod head, but had not con- 

 sidered the question of the nature of the pra^oral somites. 

 Lankester (2) was the first to suggest that (as is actually 

 the fact in the Nauplius larva of Crustacea) the praeoral 

 somites or prosthomeres and their appendages were ances- 

 trally post-oral, but have become prseoral " by adaptational 

 shifting of the oral aperture. ^^ This has proved to be a sound 

 hypothesis, and is now accepted as the basis upon which the 

 Arthropod head must be interpreted (see Korschelt and 

 Heider [3]). Further, the morphologists of the ^fifties 

 appear, with few exceptions, to have accepted a preliminary 

 scheme with regard to the Arthropod head and Arthropod 

 segmentation generally, which was misleading and caused 

 them to adopt forced conclusions and interpretations. It 

 was conceived by Huxley, among others, that the same 

 number of cephalic somites would bo found to be character- 

 istic of all the diverse classes of Arthropoda, and that the 

 somites not only of the head, but of the various regions of 

 the body, could be closely compared in their numerical 

 sequence in classes so distinct as the Hexapods, Crustaceans, 

 and Arachnids. 



The view which it now appears necessary to take is, on the 

 contrary, this — viz. that all the Arthropoda are to be traced 

 to a common ancestor resembling a Chtetopod worm, but 

 differing from it in having lost its chietse and ni having a 

 prosthomere in front of the mouth (instead of prostomium 

 only) and a pair of hemignaths (mandibles) on the parapodia 

 of the buccal somite. From this ancestor Arthropods with 

 heads of varying degrees of complexity have been developed 



