AQUATIC EARTHWORMS 



635 



gives rise to a definite number of new somites (five in most species 

 of Naididae), which form the anterior part of the posterior daughter- 

 worm. The daughter-worms, before separation, may in turn de- 

 velop budding zones, and in some cases even a third series oi these 

 zones may appear and 

 thus give rise to chains 

 of incipient individuals, 

 or zooids. In some spe- 

 cies ehains of eight zooids 

 are of ordinary occur- 

 rence. In the genus 

 Chaetogaster the plane 

 of division is in a septum 

 between two somites. 

 Although many aquat- 



\r Olicrnrhaptn Vin\7-p tViP ^^^- 9^°- Development of budding zones in Stylaria lacuslris. 

 IL WilgUeildeLcl nave Uie ^, An early stage. 5. A later stage. C. Still later sI;,kc with 



power to regenerate a second budding zone weU started. X. 5. (.-Vfter UuckarU 



missing parts, greatly developed, there is lack of evidence that it is 

 of much importance in normal reproduction. 



Environmental Relations. The well-known investigations of 

 Darwin and others, on the action of terrestrial earth worm.s on the 

 soil and its organic contents, have led to a general ai)preciation of 

 the importance of the relations of these animals to their surround- 

 ings. It is less generally understood that their aquatic relatives 

 play a very important part in reducing the great masses of aquatic 

 vegetation to a finely-comminuted condition. Oligochaeta of 

 various species abound in the mud at the bottom and along the 

 shores of most bodies of fresh water, and an almost continuous 

 stream of this mud with its decaying organic contents is passing 

 through their bodies and being still further subdivided and de- 

 prived of organic material and its available energy. Numerous 

 other species swarm in the decaying leaves and stems of coarse 

 vegetation of swampy areas and materially aid in their disintegra- 

 tion, while still other kinds populate the floating masses of algae, 

 which they rapidly devour as decay progresses. Since under 

 favorable conditions it requires but two or three days for Xauh- 

 form worms to reproduce by budding, they multiply with such 



