RELATION OF THE NEMERTEA TO THE VERTEBRATA. 607 



which is SO vital in any consideration of the ancestry of the 

 Vertebrates, viz. the origin of raetameric segmentation, it 

 appears to me that the Nemertea offer points very worthy of 

 consideration. The question of the proboscis and its sheath, 

 as comparable to hypophysis and notochord was fully treated 

 by me in another paper,i and will here only be very briefly 

 touched upon. In my opinion, this comparison is all the more 

 forced upon us, now that in other respects (nervous system, &c.) 

 new evidence of genetic relationship is here brought forward. 



The first point I wish to consider is that of metameric seg- 

 mentation. It has been specially treated of late years by 

 various authors of renown, with whom I do not wish to enter 

 at this moment into any lengthy controversy, but will briefly 

 state what may be gathered for the theory in general, from 

 a careful consideration of the incipient metamery of the 

 Nemertea. 



If we start from a more or less radiate ancestor of the 

 earliest diploblastic type, in which neither a radial nor a serial 

 repetition of organs or organ systems has yet come about, and 

 which may indifferently be considered to resemble either a 

 more flattened Trichoplax or a more spherical gastrula, we 

 may assume that in the course of the development of other 

 internal organs (towards the formation of which the secondary 

 accumulation of cells between the two primary layers often so 

 largely contributes) the radial symmetry may either be further 

 accentuated or may be replaced by a tendency towards bilateral 

 symmetry. In the latter case we are inclined to ascribe the 

 first impulse towards this bilateral symmetry to a preference, 

 which slowly establishes itself in the animal mechanism, for 

 moving in one direction rather than in any other, i. e. for 

 generally stretching forward, when moving about, one particular 

 portion of the body. 



One impulse of this sort will suffice to lead us to understand, 

 or rather to deduce, a very considerable number of conse- 

 quences, which cannot fail to make their appearance under the 



1 " On the Ancestral Forms of the Chordata," ' Quart. Journ. Micr. Sci.,' 

 vol. xxiii, 1883. 



