But in the worms split lengthwise the head in the angle may be formed at a level much farther posteriorly than the eyes. If the split extends into the head, then the eyes that develop are the supplements of those of the old part. Our analysis leads, therefore, to the conclusion that the heads, or parts of heads, in the split worms are not heteromorphic structures but supplementary heads. CHAPTER VIII SELF-DIVISION AND REGENERATION. BUDDING AND REGENERATION. AUTOTOMY. THEORY OF AUTOTOMY Self-division, as a means of propagation, is of widespread occurrence in the animal kingdom. In some cases the animal simply breaks into pieces and subsequently regeneration takes place in the same way as when the animal is cut into pieces by artificial means. In other cases the parts are gradually separated, and during this time new parts are formed by a process resembling that of regeneration after separation. A few zoologists have tried to show how the process of regeneration before separation has been derived from regeneration following self-division. It is our purpose to examine here the evidence in favor of this hypothesis. A study of the forms that propagate by means of self-division shows that the process is present in many groups of the animal kingdom. In the unicellular forms this method is universally present; and in the multicellular forms the division of the individual cells is looked upon as a process similar to the method of propagation in the protozoa. The sponges do not multiply by self-division. In the cœlenterates, on the other hand, we find this mode of propagation present in most forms. Hydra appears rarely, if at all, to divide by a cross-division, and, although one or two cases of longitudinal division have been described, it is not improbable that they have been started by the accidental splitting of the oral end. The hydromedusæ, _Stomobrachium mirabile_, _Phialidium variabile_, _Gastroblasta Raffælei_, are known to increase by division.[66] Several actinians and many corals divide longitudinally, while the scyphistoma of the scyphomedusæ produce free-swimming ephyras by cross-divisions of the fixed strobila stage. The ctenophors do not divide. It is known that several fresh-water planarians propagate by division, the tail-end breaking off in the region behind the old pharynx. In one form,[67] and possibly in others, regeneration may begin before the separation takes place. Many of the rhabdocœlous planarians increase by cross-division--the separation taking place more nearly in the middle of the body. In these forms the parts develop new organs more or less completely before they separate. In the trematodes self-division does not take place. The division of the body of the tapeworm into proglottids may represent a process of self-division, but the proglottids do not regenerate after separation.