RELATION OF THE NEMERTEA TO THE VERTEBRATA. 617 



more perfect evolutional phases of the process, which are at 

 the same time those that have till now been more carefully 

 investigated, the new head is formed anteriorly to the rupture, 

 or at least its essential parts are pre-established." 



My own views emphasize the presence of a peculiar process 

 of development of the internal organs, running parallel to this 

 predisposition for rupture in a particular spot — the spot 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 Coeleaterata, and at the same 

 time predecessors of both Chordata and Appendiculata.^ 



This view of the origin of metamery also affords an explana- 

 tion 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 pecu- 

 liarity) immediately 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, each 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 ex- 

 ternal agents, has been of very great moment in the bringing 



* Gegenbaur, in his ' Grundriss der Vergleichendeii Auatomie' (1878), hints 

 at similar explanations to those advocated by Emery and myself, when he says 

 (p. 64) : — " Die Metamerie . . . lasst Zustande des Beginnes und der 

 nicht ausgefiihrten Beendigung mannichfach erkennen .... In dem 

 Maasse als ein Metamer die Abhangigkeit vom Gesammtorganismus durch die 

 Ausbildung seiner eigenen Organe aufgiebt emancipirt er sich vom Ganzen 

 und gewinnt die Befahigung freier Existenz." Further on he speaks of inci- 

 pient metamery as " eine stellenweise, fiir den Organismus praktisch werdende 

 Ausbildung " of the different organ systems. 



