HI. ANNELIDA: CH^TOPODA. 



309 



always arise from the ccelomic epithelium. In the Oligochaeta 

 (fig. 286), besides the nephridia in the genital segments special 

 oviducts and vasa deferentia occur which are usually regarded as 

 modified nephridia. 



Of 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 

 or fewer somites. In many Oligochsetes they may empty into the anterior 

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

 Nephthys, Goniadd) 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 larvae occur. These, in spite of their many modifications, 

 are comparable with the < Loven's larva/ the trochophore already 



mes 



FIG. 278. .4, larva of Polygordiu*; B, same changing to segmented worm. (After 

 Hatscnek.) a, anus; Ten, excretory organ; mes, segmented mesoderm. 



described (p. 306). The differences largely consist of modifica- 

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

 increased (polytroche larvae), 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 

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



