This corresponds to the behaviour of their enemies, which may often bite off a piece or tear it away when the worm attempts to escape, but never cut it up into pieces. The regenerative capacity is more highly developed in the genus _Allurus_, more highly still in the worms of the genus _Criodrilus_ which lives in the mud at the bottom of lakes, and most highly of all in the genus _Lumbriculus_ which lives at the bottom of small ponds. Long ago Bonnet cut up a specimen of _Lumbriculus_ into twenty-six pieces, of about two millimetres in length, and he observed most of these grow to complete worms again. His experiments have often been repeated in recent times, and have been extended and made more precise in many ways. Von Bülow was able to get whole animals from pieces consisting of from four to five somatic segments, and with eight or nine segments he almost invariably succeeded. A _Lumbriculus_ which he had cut into fourteen pieces, one of which only measured 3.5 mm. in length, gave rise to thirteen complete worms with head and tail; only one piece perished. These worms have little enemies with sharp jaws which may gnaw at them behind or before but cannot swallow them whole. Lyonet, famous for his analytic dissection of the wood-caterpillar (_Cossus ligniperda_), observed when he was feeding the larvæ of dragon-flies with these Lumbriculid worms that 'the anterior end of some whose posterior end had been gnawed away by the larvæ continued to live on the ground.' We can thus understand why a high power of regeneration is of use to these worms, and at the same time why it is advantageous to them to contract so that they break in pieces on very slight irritation, but to this we shall refer again. The very diverse potency of the faculty of regeneration in animals belonging to the same small group, and nearly, if not quite, at the same level of organization, seems to show clearly that we have here to do with adaptation to different conditions of life, although we cannot demonstrate this in detail. It would certainly be erroneous to regard the conditions of life as uniform, since the worms in question not only live in different places--in the earth, in mud, or in water--and are thus exposed to different enemies, and since they may also be quite different in regard to size and speed, in means of defence, and possibly also of defiance, as is indeed in some measure demonstrable. We meet with the same thing in a group of still smaller worms, Rösel's 'water-snakelets,' species of the genus _Nais_. These, too, behave in a variety of ways in the matter of regeneration, for while many species, such as _Nais proboscidea_ and _Nais serpentina_ will, if cut into two or three pieces, become two or three worms respectively, Bonnet expressly mentions an unnamed species of _Nais_ which does not bear cutting up at all, and even dies if its head be cut off.