ANNUL ATA 235 



involves the reduction in size of the enormous anterior region, 

 and the growth of segments at the posterior end, and is char- 

 acteristic of Polychaeta. 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 sexual reproduction many worms, par- 

 ticularly the aquatic forms, have the power of multiplying by 

 fission. In some instances this may consist of a mere breaking 

 in two, as was seen to be possible in the starfish, each part 

 regenerating segments corresponding to those lost. In other 

 cases (Nais, Dero, etc.) zones of rapidly forming segments 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. 101, z'). 



In some of the Polychaeta (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 marine, and the 

 Oligochaeta are chiefly fresh water or terrestrial in habit. A 

 few of the latter 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, 



