122 THE VOYAGE OF H.M.S. CHALLENGEE. 



arrangement of Balanoglossus amongst the Chordata, ajDpears to be quite as justifiable as 

 the elevation of the Urochorda to their new dignity in zoological classification. 



There is, however, a great difierence between looking at Balanoglossus as a low type 

 amongst the Chordata (in which I fully agree with Bateson) and rejecting the signifi- 

 cance of the Nemertean tjrpe as one of transition in the way above indicated. 



There is no doubt that the Nemertea represent a more primitive phase than the 

 Enteropneusta (Hemichorda). They have no enterocosle, and they have no gill-slits ; 

 but their nervous system shows certain unexpected analogies uith that of the higher 

 Chordata of more intrinsic value than those that obtain between Balanoglossus and the 

 Chordata in general. Also for the important question, which is so vital in any considera- 

 tion of the ancestry of the Vertebrates, viz., the origin of metameric 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 pajDer, 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 segmentation. 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 

 genei-ally stretching forward, when moving about, one particular portion of the body. 



One impulse of this sort will sufiice to lead us to understand, or rather to deduce, a 

 very considerable number of consequences, which cannot fail to make their appearance 

 under the influence of natural selection acting upon the organisms that have inherited 

 this tendency in different degrees. Thus we may understand the narrowing and 

 lengthening of an animal that moves in one direction in preference to any other ; and 

 similarly the development in the nervous system of a centralisation not far away from 

 the anterior extremity. 



