RELATION OF ARTHROPOD HEAD TO ANNELID PROSTOMIUM. 249 



the first, second, or third segment; whilst in some forms the 

 prostomium is quite rudimentary (Diacheta), in others it 

 extends backwards dorsally (as marked by a groove) to the 

 hinder limit of the peristomium (Lumbricus). 



In the Amphinomids it grows backwards over several seg- 

 ments.^ 



The peristoraial segment, to which the prostomium is at- 

 tached, is almost always considerably modified in connection 

 with the mouth, and may even be sharply marked off from the 

 posterior segments. It is often called the cephalic segment 

 (Kopf segment, segment cephalique, buccal segment, head 

 segment, &c.). The peristomium and prostomium together 

 constitute the head in Oligochaetes and some Polychaetes. 

 In many Polychaetes, however, several anterior segments may 

 become so modified as to contribute to the formation of the 

 head. 



Having thus briefly reviewed the structure and relations of 

 the prostomium in the Chsetopoda, we must proceed to a more 

 accurate study of its homology. 



The prostomium can be one of three things : (I) a modified 

 or reduced segment; (2) an incipient segment, growing on the 

 anterior surface of the peristomium ; (3) not a segment at all, 

 but a structure of diff'erent and special nature. 



Before attempting to prove that the last interpretation is 

 the true one, it must be clearly established what we mean by 

 a true segment or metamere, and then how such a metamere 

 differs radically from the prostomium. 



It is comparatively easy to give a serviceable definition of a 

 typical segment : it is a region more or less distinctly marked 

 off from the rest of the body by transverse grooves, surround- 

 ing the alimentary canal, containing a special coelomic cavity 

 (more or less completely separated off from the coelom of 

 adjoining segments by means of transverse septa), a pair of 

 nephridia and of peritoneal funnels communicating with the 

 exterior, a pair of ganglionic enlargements of the ventral 



' An excellent discussion of the structure and morphology of the prosto- 

 ipium and brain of the Polychseta has lately been given by M. Racovilza (14). 



