RELATION OP THE NEMERTEA TO THE VERTEBRATA. 613 



swallowed. So in the Nemertea there is indeed a very strong 

 faculty of spontaneous division combined with the faculty of 

 repair;^ and anybody who has observed a fresh and living 

 Cerebratulus, with its extremely delicate sense of touch, 

 commence to rupture into two, in preference at the spot where 

 it was grasped with the forceps, cannot fail to see in this a 

 defensive action. 



This mode of self-defence may in quite another respect be 

 useful to the species, because at the same time it serves for 

 propagation. Thus we see that the passage of this defensive 

 process to that of reproduction by fission is so gradual, that it 

 would be impossible to decide in every case what name should 

 properly be applied to it. It cannot well be denied that in all 

 probability ours is only a special case, in which the power of 

 reproducing the species by a process of fission, reaching down 

 as far as the unicellular ancestors, has come to be regulated 

 by other motor forces than growth, and — if it may not be called 

 voluntary fission — still may be regarded as sudden and spon- 

 taneous fission, brought about by external influences, of a 

 threatening nature to the further existence of the specimen. 

 This regulation is no doubt a consequence of selection. Schi- 

 zogony having once been established, it must have been further 

 beneficial to the species, on the grounds that were developed 

 above, that the internal organs should be present in multiple 

 numbers, and this having once come about it is easy to under- 

 stand that a regular, rigorously metamerous arrangement of 

 this multiple material, still more fully answers the same pur- 

 pose, and is gradually evolved under the influence of selection. 

 Thus we may be said to be able to follow the appearance of 

 metamery in a bilateral animal, along all the gradual steps by 

 which it is evolved, and many of these steps have remained 

 fixed and permanent in different Nemertean genera. 



' Both M'Intosh aud Barrois have observed and described very peculiar 

 cases of repair in Nemertea, where the head, brain, side-organs, &c., were 

 reproduced on a headless trunk-piece. These experiments are well worthy of 

 careful repetition. It may be that only those fragments in which a portion of 

 the oesophagus was retained were capable of repair of the head. 



