III. ANNELIDA: CHjETOPODA. 



309 



[tlwa3-s arise from the coelomic epithelium. In the Oligoehseta 

 (fig. 2SG), besides the nephridia in the genital segments special 

 oviducts and vasa defereutia occur which are usually regarded as 

 modified nephridia. 



<Jf the many modifications of nephridia only a few can be noticed here. 

 Occasionally there may be more than one pair in a somite, or they may 

 have more than one nephrostome. Again, they may be lacking from more 

 fir fewer somites. In many Oligochjetes they may empty into the anterior 

 or posterior ends of the digestive tract. In many (Glycera, Hesione, 

 Nephthys, Qoniada) the internal ends of the nephridia are branched, 

 the branches being closed by ' solenocytes,' tubular cells bearing an internal 

 bundle of cilia. 



In many marine annelids there occurs a metamorphosis in which 

 pelagic larvas occur. These, in spite of their many modifications, 

 are comparable with the ' Loven's larva,' the trochophore already 



B 



- kn 



t'lo. :?7K.— .4, larva of Pol\j<jor(Vwfs; B, same chanfrin? to sesmented worm. (After 

 Hatschek.) a, anus; kn, excretorj- organ; mc.s, segmented mesoderm. 



described (p. 30G). The differences largely consist of modifica- 

 tions of the ciliary apparatus; the number of bands may be 

 increased (polytroche larva), or a single band may occur at the 

 middle (mesotroche) or at the end (telotroche) of the body. The 

 larva becomes a segmented worm by the hinder end of the larva 

 gniwing out and dividing into segments (fig. 278). In this 



