274 



COiLlIELMINTHES 



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

 also breaks down and the ccelomic cavities miite into one. The nephridia 

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

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

 the adult. 



B 



kn 



Fig. 257. — . 1, larva o£ Polygonlius; P, same changing to segmented worm (after 

 Hatschek). a, anus; kn, excretory organ; mcs, seginentcJ mesoilerm. 



The land and fresh-water annelids develop directlv, 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 

 Rolifera the farther conclusion is drawn that the annelids ha\e descended from 

 rotifer-like ancestors, the body ca\ily, nelihridia, blood-vessels, and ventral 

 nerve chain being new formations. 



Besides sexual reproduction maitv fresh-water and marine species 

 reproduce asc-xually, tliis being possible from the great honiotroray of the 

 segmentation. By rapiil growth at the liinder end as well as at a more 

 anterior budding zone numerous somites are formed, wltich separate in 

 groups from the piareirt to form voutrg worms. In some cases the forma- 

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

 result being chains of worms (tig. 25S) wliich in some instances branch. 



T!y a combination of sexual and asexual reproduction a tyjilcal alternation 

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



