[Illustration: FIG. 1.--_A_-_B_. After Trembley, _C_-_G’_. After Bonnet. _A._ Seven-headed hydra made by splitting head-ends lengthwise. _B._ Illustrating the method of turning hydra inside out by means of a bristle: 1, foot being pushed through mouth; 2, completion of process. _C._ Middle piece of an earthworm (cut into three pieces) with new head and tail. _D._ Anterior part of an earthworm regenerating a new “delicate” tail. _E._ Posterior third of a worm (lumbriculus) that regenerated two heads. _F._ Middle piece of a worm (another species) cut into three pieces. It made a tail at each end. _F’._ Anterior, enlarged end (tail) of last. _G._ Small piece of a worm. _G’._ Regeneration of head and tail of same.] worms, one of which, at least, seems to have been the annelid lumbriculus. His first experiments (1741) showed that when the worm is cut in two pieces, a new tail develops at the posterior end of the anterior piece, and a new head at the anterior end of the posterior piece. He found that if a worm is cut into three, four, eight, ten, or even fourteen pieces, each piece produces a new worm; a new head appearing on the anterior end of each piece, and a new tail on the posterior end (Fig. 1, _G_, _G’_). The growth of the new head is limited in all cases to the formation of a few segments, but the new tail continues to grow longer, new segments being intercalated just in front of the end-piece that contains the anal opening. In summer the regeneration of a new part takes place in two to three days; in winter in ten to twelve days, this difference not being due to the time of year, but to the temperature. Bonnet found that if a newly regenerated head is cut off, a new one regenerates, and if the second one is removed, a third, new one develops, and in one case this occurred eight times: the ninth time only a bud-like outgrowth was formed. In other cases a new head was produced a few more times, but never more than twelve. He thought that the capacity of a part to regenerate is in proportion to the number of times that the animal is liable to be injured under natural conditions. Bonnet found that short pieces from the anterior or posterior end of the body failed to regenerate, and usually died in a few days. Occasionally two new heads appeared at the anterior end of a piece (Fig. 1, _E_), and sometimes two tails at the posterior end. Another kind of fresh-water worm[2] was found that gave a very remarkable result. If it was cut in two pieces, the posterior piece produced at its anterior end, not a new head, but a new tail. Thus there is formed a worm with two tails turned in opposite directions, as shown in Fig. 1, _F_, _F’_.