274 



CGELHELMINTHES 



disappear in the adult. In many worms the septa between the somites 

 also breaks down and the coelomic cavities unite into one. The nephridia 

 also arise independently of the protonephridial system, which is often called 

 head kidney because the chief part of the trochophore forms the head of 

 the adult. 



B 



kn 



mes 



mes 



FIG. 257. A, larva of Polygordius; B, same changing to segmented worm (after 

 Hatschek). a, anus; kn, excretory organ; mes, segmented mesoderm. 



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

 sess a reminiscence of a larva in that the head lobes are very apparent and 

 contain protonephridia, which leads to the conclusion that these animals earlier 

 had a metamorphosis. From the resemblance of the trochophore to the 

 Rotifera the farther conclusion 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 

 reproduce asexually, this being possible from the great homonomy of the 

 segmentation. By rapid growth at the hinder end as well as at a more 

 anterior budding zone numerous somites are formed, which separate in 

 groups from the parent to form young worms. In some cases the forma- 

 tion of new somites may take place more rapidly than the separation, the 

 result being chains of worms (fig. 258) which in some instances branch. 



By a combination of sexual and asexual reproduction a typical alternation 

 of generations occurs, the origin of which receives light from the following facts: 



