EELATION OF THE NEMERTEA TO THE VERTEBRATA. 6l5 



organs is in a definite relation to these phenomena — which 

 might also deserve the name of incipient metamery — in the 

 muscular layers. 



For the present the fact is^ however, not yet definitely demon- 

 strated that these successive blocks are indeed present as such 

 in the living animal. The possibility is still open that they may 

 be waves of contraction which have been fixed at the moment 

 of the immersion of the animal in the preserving fluid. For 

 this reason I will not lay any undue weight on this observation. 



The ideas concerning the origin of metamery here expressed, 

 and advocated for several years in my university lectures, difi*er 

 from those of Lang (loc. cit.) and Sedgwick,^ in so far as 

 they do not recognise the primary importance of the so-called 

 coelomic sacs — the paired archenteric diverticula ofAmphioxus 

 — for the solution of this question. 



The question of the Vertebrate coelome, so full of obscurities 

 and difficulties, is purposely left out of consideration here, where 

 the relation to archiccelous ancestral forms is discussed, and 

 where an attempt is made to show that it is indeed probable 

 that the impulse towards the establishment of metamery is due 

 to forces for which the archenteron was not the only, nor per- 

 haps the most important part of the organism to act upon. 



Still more diff'erent are they from those advocated by Perrier^ 

 and Cattaneo,^ who have adhered to and extended the idea 

 already held by others, but by them most actively defended, 

 " that the metamery of Arthropods, Vertebrates, and a great 

 many Vermes, has originated out of the multiplication by 

 transverse fission of very simple primitive worms which 

 were not metamerous. The products of this transverse 

 fission remaining connected together have then formed a 

 chain of individuals, or a linear colony ; later on the unity 

 of the chain has become more definitely established, the 

 single individuals at the same time becoming diff'erent both in 



' A. Sedgwick, " On the Origin of Metameric Segmentation," ' Quart. 

 Journ. Micr. Sci.,' vol. xxiv, p. 43, 1884. 



^ E. Perrier, 'Les colonies auimales,' Paris, 1881. 



^ G. Cattaneo, 'Le colonic lineari e la morfologia dei MoUusclii,' Milano, 

 1883. 



VOL. XXVIl, PART 4. NEW SER. U U 



