310 



CCELIIELMINTHES. 



jointed portion the coelom arises as a new formation, divided from 

 the first into separate chambers. The nephridia also arise de 

 novo, independent of the protonephridial system, which is often 

 called head kidney because the chief part of the trochophore forms 

 the head of the adult. 



The fresh-water annelids develop directly, but the embryos pos- 

 sess a reminiscence of an earlier larval life in that the head lobes are 

 very apparent and contain protonephridia, which leads to the con- 

 clusion that these animals earlier had a metamorphosis. From the 

 resemblance of the trochophore to the Rotifera the farther conclu- 

 sion is drawn that the annelids have descended from Rotifer-like 

 ancestors, the body cavity, nephridia, blood-vessels, and ventral 

 nerve chain being new formations. 



Besides sexual reproduction many fresh-water and marine 

 species may reproduce asexually, this being rendered possible by 

 the great homonymy of the segmentation. By rapid growth at 

 the hinder end as well as at a more anterior budding zone numer- 

 ous somites are formed which separate in groups from the parent 

 to form young worms. In some cases the formation of new somites 



FIG. 279. Budding in Myrianida. (After Milne-Edwards.) The sequence of letters 

 shows the ages of the individuals. 



may take place more rapidly than the separation, the result being 

 chains of worms (fig. 279). 



By a combination of sexual and asexual reproduction a typical alter- 

 nation of generations occurs, the origin of which receives light from the 

 following facts: In many polychaBtes which reproduce exclusively by the 

 sexual process the sexless slowly-moving young (atoke) at sexual ma- 

 turity becomes so altered in appearance as to have been described under 

 another name. It becomes very active in its movements, and the hinder 



