That regeneration of this kind could have been arranged for if it had been useful we learn from the Planarians among the flat worms, in which every piece cut out of the body, large or very small, from the middle, from the left side, or from the right side of the animal, grows into a complete Planarian. The animal can be halved longitudinally, as in Fig. 97, and each half will grow to a whole. This again is quite intelligible from the biological point of view, for these flat, soft, and easily torn animals are exposed to all sorts of injuries, and are, in point of fact, frequently mutilated by enemies which are unable to swallow them whole. Von Graaf not infrequently found examples of marine Planarians (_Macrostomum_) which lacked 'a part of the posterior end or the whole tail region as far as the food-canal,' and of species of _Monotus_ he found 'very often' in May specimens with the posterior end split or broken off. Probably the persecutors of these flat-worms are some species of Crustacean, but, at any rate, so much is proved, that the Planarians have abundant opportunities of making use of their faculty of regeneration, and that the species gains an advantage from it in respect to its preservation. [Illustration: FIG. 97. _A_, a Planarian, which has been divided into two by a longitudinal cut. Each half can grow into an entire animal. _B_, the left half at the beginning of the regenerative process. _C_, the same completed. After Morgan.] In contrast to this, worms which live within other animals, and are thus secure from mutilation, such as the familiar round-worms (_Nematoda_), have no power of regeneration at all, and do not survive either longitudinal or transverse division.