14 Alexnudcr Petrnukivitch, 



eleventh edition of the Encyclopaedia Britannica, Lankester himself 

 writes. 



"According to older views the increase in the number of somites 

 in front of the mouth would have been regarded as a case of inter- 

 calation by new somite-budding of new prae-oral somites in the 

 series. We are prohibited by a general consideration of metamerism 

 in the Arthropoda from adopting the hypothesis of intercalation 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 i and 2 bore the man- 

 dibles lost those mandibles, developed their rami as tactile organs, 

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

 previous jaw-bearing function was 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 speciali- 

 zation. 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." 



In discussing the validity of Lankester's system which unquestion- 

 ably has much in its favor, we have to subject the following questions 

 to a careful scrutiny : (i) Do we possess any evidence that post-oral 

 somites (opisthomeres) have passed forward and become prostho- 

 meres ? (2) What evidence can be brought in support of the assertion 

 that both grades of Lankester's class Arachnida have two prostho- 

 meres, while the Crustaceae, Chilopoda and Hexapoda have three 

 prosthomeres ? (3) Is it right to assume that the appendages of the 

 somites which passed in front of the mouth have changed their func- 

 tion as jaws to become tactile organs, or is the second alternative of 

 Lankester's more correct, "that the buccal gnathobasic parapodia 

 (the mandibles) 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 prae-oral region had 

 taken ];lace." ? (4) Is the segmentation of the head of fundamental. 



