STRQCTDRE AND CLASSIFICATION OP THE AliTHROPODA. 575 



According to older views the increase of the number of 

 somites in front of the mouth would have been regarded as a 

 case of intercalation by new somite-budding of new prteoral 

 somites in the series. We are prohibited by a general con- 

 sideration of metamerism in the Arthropoda (see a previous 

 section of this article) from adopting the hypothesis of inter- 

 calation of somites. However strange it may seem, we have 

 to suppose that one by one in the course of long historical 

 evolution somites have passed forwards and the mouth has 

 passed backwards. In fact, we have to suppose that the 

 actual somite which in grades 1 and 2 bore the mandibles 

 lost those mandibles, developed their rami as tactile oro-ans 

 and came to occupy a position in front of the mouth, whilst 

 its previous jaw-bearing function Avas taken up by the next 

 somite in order, into which the oral aperture had passed. A 

 similar history must have been slowly brought about when 

 this second mandibulate somite in its turn became agnathous 

 and passed in front of the mouth. The mandibular parapodia 

 may be supposed during the successive stages of this history 

 to have had, from the first, well-developed rami (one or two) 

 of a palp-like form, so that the change required when the 

 mouth passed away from them would merely consist in the 

 suppression of the gnathobase. The solid palpless mandible 

 such as we now see in some Arthropoda is, necessarily, a late 

 specialisation. Moreover it appears probable that the first 

 somite never had its parapodia modified as jaws, but became 

 a prosthomere with tactile appendages before parapodial 

 jaws were developed at all, or rather pari passu with their 

 development on the second somite. It is worthwhile bearino- 

 in mind a second possibility as to the history of the prostho- 

 meres, viz. that the buccal gnathobasic parapodia (the man- 

 dibles) were in each of the three grades of prosthomerism 

 only developed after the recession of the mouth and the 

 addition of one, of two, or of three post-oral somites to the 

 prteoral region had taken place. In fact, we may imagine 

 that the characteristic adaptation of one or more pairs of 

 post-oral parapodia to the purposes of the month as jaws did 



