10 THE SHORTER SCIENTIFIC PAPERS 



tera (Passalus, etc.), occurring at the same time with the 

 trochantin, proves that it is distinct from the latter. 



The origin of the three pieces, coxa genuina, meron, and 

 trochantin, the relative positions of which are shown in 

 the accompanying figures, is more difficult of explanation. 

 A striking characteristic of the Hexapoda and Chilopoda 

 is the more or less complete fusion of the first two {coxa 

 genuina and meron) into the so-called coxa. Theoretical- 

 ly the episternum^ and epimeron may be regarded as cor- 

 responding basal segments of these fused pieces, and taking 

 into consideration the apparent absence of a piece homol- 

 ogous with the meron in the Diplopoda, we are led to the 

 interesting inference that this group represents a class in 

 which each pmnitive metamere bears a pair of appendages, 

 while the Chilopoda and Hexapoda represent a widely 

 divergent class, in which two primitive -metameres have 

 attained a more complete fusion^ the rudim^entary ap- 

 pendage belonging to each posterior segment fusing with 

 the base of the functional appendage belonging to the 

 anterior tnetamere.^ The posterior appendage would 

 then be represented by the meron and epimeron.* In 

 this case we can refer the origin of coxa genuina and 

 episternum, as well as the meron and epimeron, to the 

 same causes v/hich produce segmentation in the append- 

 age. The trochantin appears to be nothing more than a 

 part of the antecoxal piece, a lateral prolongation of which 

 became constricted off in a primitive form. Again we 

 are confronted with the question as to the origin of the 

 appendages in the Arthropoda, whether they are ventral 

 or dorsal parapodia, or a fusion of the two, as in Nereis, or 

 whether the origin was entirely independent of the para- 

 podia. A study of Peripatus appears to throw no light 



^Frequently the episternum appears to be cut off from the coxa by the sternum 

 or by the trochantin, but in such cases the coxa is usually prolonged internally, so 

 that it meets the episternum. 



^At present there appears to be better evidence for believing that these pieces 

 indicate a fusion of the segments than to hold that an exopodite and entopodite are 

 represented. 



^A study of the position and homologies of certain of the pleural and dorsal 

 sderites in Hexapoda and Chilopoda appears to confirm this inference. 



■*While thus far the majority of embryological evidence appears to be against this 

 view, it seems possible that secondary modifications have caused a misinterpretation 

 of the lines of descent. 



