PHYLUM ANNULATA 



441 



temporary character. These have been found to occur in the head 

 (prostomium) of many larval Oligochasta and Polychteta. They 

 are ciliated intra-cellular tubes, sometimes branched, which do 

 not open into the cavity of the prostomium. Sometimes ciliary 

 names projecting into the lumen occur at the inner ends of the 

 branches or of the undivided tube. Embryonic nephridia have 

 also been shown to occur in the body in certain forms. 



In the arrangement of the reproductive organs in the 

 Cha?topoda there is an essential difference between the two sub- 

 classes, the Oligochaeta being hermaphrodite, and the Polychaeta, 



refir.c 



fterlb 



vent.vess 



Jbe.rU:- 



B 



vent, vzss 



epryl 



jberib 



vent, yeas 



v&rit. v&ss 



FIG. 345. Diagram to illustrate the development of a gonad from the peritoneal (ccelomic) epithelium 

 in one of the Polychasta. pn-it. peritoneal membrane ; i\pr. <jl. gonad (reproductive organ) ; 

 vent. I'*.**, ventral vessel. (After E. Meyer.) 



with only a very few exceptions, unisexual. In the latter the 

 gonads, ovaries or testes as the case may be, are masses of cells 

 which are developed as the result of a sort of proliferation of the 

 coelomic epithelium in certain positions (Fig. 345). Usually these 

 organs, which are only conspicuous about the breeding season, occur 

 in the great majority of the segments of the body ; sometimes they 

 are confined to a certain region. The exact place which they 

 occupy in the interior of the segment varies in different cases. 

 Sometimes they surround one of the principal blood-vessels; 

 sometimes they are situated laterally, in the bases of the para- 

 podia. The sperms frequently undergo the final stages in their 

 development after they have become detached from the testes 



