REPORT ON THE NEMERTEA. 129 



My own views emphasise the presence of a peculiar process of developroent of the 

 internal organs, running parallel to this predisposition for rupture in a particular spot 

 — the vspot which will correspond to the outwardly visible demarcation between the future 

 segments. They thus go one step further — and, in my opinion, a very essential step — 

 in the attempt to explain the origin of metamery in the lower Platyelminthes, these 

 bilateral descendants of radiate Coelenterata, and at the same time predecessors of both 

 Chordata and Appendiculata.' 



This view of the origin of metamery also aflfords an explanation for the very different 

 degrees in which we find metamery or segmentation expressed in the different divisions 

 of the animal kingdom. The incipient metamery which we have traced (and which we 

 have pictured to ourselves as arising through natural selection amongst those forms, 

 which, while developing in length, find metamery to be a protective peculiarity) imme- 

 diately creates, by the fact of its existence, new and variable material for selection, 

 again to be acted upon. And whilst metamery develops in one direction in one line of 

 descendants, the other line brings to the foreground a different set of advantageous 

 combinations, eaeh of them again the stock of new and varied forms. In other words, 

 metamery once established in its most primitive form, and intimately connected with 

 spontaneous fission under the influence of external agents, has been of very great 

 moment in the bringing about of new and endless variations of animal life. And it is 

 irrational, when we have before us, say one of the lowest Vertebrata, in which nobody 

 will deny the presence of distinct metameric segmentation,, to conclude that this metamery 

 must necessarily be in many respects reduced, and that in the ancestral forms it must 

 have been far more complete, must have stretched forwards along the whole of the head, 

 must have been more forcibly expressed than it is now — in all the cephalic nerves, in 

 the nephridia, the gill-slits, &c. ; — all this on the presumption of the existence of an 

 ancestor so completely and exemplarily segmental as to throw no light on the origin of 

 segmentation and metamery, unless by the aid of Perrier's and Cattaneo's exaggerations. 

 Such conclusions must, however, necessarily be made by those who follow Dohrn's and 

 Semper's lead concerning the phylogeny of the Chordata. 



Bateson, in taking Balanoglossus as his starting-point, finds the acknowledged points 

 of resemblance in the metamerous gill-slits, &c., and adds to them important data con- 

 cerning the metamerous coelomic diverticula. Still, for a general view on the origin of 

 metamery, Balanoglossus offers no points that we do not find more strongly represented 

 and more forcibly expressed in the Nemertea. It certainly deserves mention that long 



1 Gegenbaur, in his Grundriss der Vergleichenden Anatomie (1878), hints at similar explanations to those advocated by- 

 Emery and myself, when he says (p. 64) :— " Die Metamerie .... liisst Zustande des Beginnes und der nicht aus- 

 gefiihrten Beendigung mannichfach erkenneu .... In dem Maasse als ein Metamer die Abhaugigkeit vom Gesammt- 

 organismus durch die Ausbildung seiner eigenen Organe aufgiebt eniancipirt er sich vom Ganzeu und gewinnt die 

 Bei^ibigung freier Existenz." Further on he speaks of incipient metamery as " eine stellenweise, fur den Organismus 

 praktisch werdende AusbUdung" of the different organ systems. 



(ZOOL. CHALL. EXP. — PART LIV. 1887.) • Yi\l\\ 17 



