236 ZOOLOGY 



and is characteristic of Polychseta. The development of leeches 

 is direct as in the Oligochaeta, or in some instances it might be 

 more accurate to say that the process of metamorphosis is very 

 much abbreviated, being completed by the time of hatching. 



274. In addition to sexilal reproduction many worms, par- 

 ticularly the aquatic forms, have the power of multiplying by 

 budding. Zones of rapidly forming segments {Nais, Dero, etc.) 

 are produced somewhere in the mid-region of the body, and from 

 this zone a new head is developed for the posterior zooid and a 

 new tail for the anterior zooid, which usually become structurally 

 complete before the separation takes place (Fig. loi, z'). 



In some of the Polychseta (as Autolytus) a distinct alternation 

 of generation is found in which sexual and non-sexual individuals 

 are of very different appearance. 



When artificially mutilated the earthworm, and some other 

 types as well, may regenerate the lost portions. Groups of 

 segments of one worm may be grafted upon another, complete 

 healing taking place in such a way as to produce an apparently 

 normal worm. Pieces may be grafted on the side of another 

 worm in such a way as to produce a forked or otherwise 

 abnormal result. 



275. Ecology. — The leeches are aquatic in habit and 

 many of them live on the blood of higher animals, — a kind 

 of temporary parasitism; the Polychaeta are almost exclusively 

 marine, and the OligochEeta are chiefly fresh water or terrestrial 

 in habit. A few of both groups are parasitic. Of the aquatic 

 worms some are actively free-swimming, others crawl in and out 

 among the living and dead matter of the bottom, others bur- 

 row in the sand, or secrete a tubular skeleton into which they 

 may retire. Their chief economic importance is that they serve 

 as food for fish and other food-animals. The earthworm, in 

 forming its underground burrows, eats its way into the earth, 

 swallowing the soil for the organic matter which it contains 

 and passing it through its digestive tract. These castings 

 may often be seen at the mouth of the burrows. Worms thus 

 break up the soil, making it more porous and accessible to air, 

 moisture, bacteria, and the rootlets of plants. Darwin esti- 



