252 



EDWIN S. GOODRICH. 



sliowu that a pair of somites are formed in the peristomiura, 

 become hollowed out, and even give rise to peritoneal funnels 

 (E. Meyer, 11). Nephridia are almost invariably developed 

 in this segment. In Polychaetes, moreover, a pair of lateral 

 appendages are often developed, although they generally 



Pt.e... 



Fig. 4. 



Fig. 3. — Ventral view of an embryo of Allolobophora putra (after 



Vejdovsky). 

 Fig. 4. — Diagrammatic plan of the anterior segments of the Oligochaeta. 



Pr. I. Procephalic lobe ; other letters as in Figs. 1 and 2. 



become highly modified. In fact, it becomes evident, when 

 we examine the development and the adult structure of the 

 peristomium in the various groups of the Annelids, that it is 

 really a raetamere strictly comparable to the posterior seg- 

 ments, even when much modified owing to its position at the 

 anterior end of the animal. 



The prostomiura, on the contrary, presents none of the 

 characters essential to a segment. It never surrounds the 

 alimentary canal ; it never possesses a pair of mesoblastic 

 somites.^ The cavity which it contains is primitively of the 



* Considerable confusion has been introduced into this question, apparently 

 by the misunderstanding of Kleinenberg's results (6). The terms "cephalic 

 germinal streak," "head segment," "head cavity," "cephalic zoonite," used 

 by that author all seem to refer to the peristomium (not prostomium). 

 Describing the development of the first pair of somites, he says, " The 



