224 



L WER IN VER TEBRA TES. 



breaks, and there are now two worms, one with a new tail, the other with a new head. 

 In Nais it is very common to see several "budding zones" at once in various stages 

 of develoj)ment. But the process of division does not always proceed in precisely 

 the same way. The genus JLumhriculus, one of the Tubificidae, also multiplies by 

 transverse division, but it breaks in two first, and then develops the germinating tissue 

 out of which the missing parts are redeveloped for each individual, a new tail for the 

 front, and a new head for the hinder of the two. The new hind end arises as a little 

 bud, consisting of new cells and ciliated over its surface, and subsequently forms the 

 requisite number of segments. If the head is cut off it is reformed — a most conve- 



FiG. 215. — Nais probosoidea reproducing by fission, enlarged. 



nient arrangement, which, were it feasible with man, would essentially diminish the 

 inconveniences of capital punishment. 



The delicate fresh-water annelids are much preyed upon by carnivorous insect 

 larvae, and it is not uncommon to see a Dijtiscus larva, for instance, seize one in its 

 jaws and snijj the poor worm in two, one half escaping. This mishap, which would 

 be fatal to most animals, is only a temporary inconvenience to a JVais or Ziumbriculus. 

 It is evident that their extraordinary reproductive endowments must be one of the 

 most imjjortant factors in the preservation of the species. Bonnet, one of the most 

 accurate of observers, found that the process of regeneration is completed within a 

 week, and proved that one worm, divided into several pieces, might produce, under 

 favorable circumstances, an equal number of new complete individuals, so that some- 

 times the very act of destruction, as in the fabled hydra, multiplies instead of annihi- 

 lating the victim. 



