GENERAL CONSIDERATIONS. 



In venturing at the close of this Report on the Nemertea, collected by H.M.S. 

 Challenger, to leave the region of demonstrated facts and actual observations, and to 

 enter upon that of speculation and suggestion, I gladly avail myself of the permission for so 

 doing granted to me by the editor, Mr. John Murray. I thought it necessary to ask for 

 that permission, because general speculations on the ancestry of the Chordata hardly 

 appeared to me to fit into the framework of these Eeports. My desire in this case to 

 deviate from a rule which I held to be salutary, was due to the fact that of late these 

 speculations have been conducted along very varying channels, an entirely new one 

 having only very lately been opened by Bateson's important series of papers on Bal- 

 anoglossus. An attempt to give more depth to one of these channels, and thus to lead 

 into it the attention of a greater number of my fellow-workers, especially commended 

 itself to me, since it was my conviction that the lines laid down by myself in former 

 publications derived considerable support from the Challenger material, and were thus 

 entitled to a renewed and full consideration. 



I would formulate the proposition, to the further development of which this chapter is 

 to be devoted, as follows : — 



More than any other class of invertebrate animals, the Nemertea have preserved 

 in their organisation traces of such features as must have been characteristic of those 

 animal forms, by which a transition has been gradually brought about from the archi- 

 ccelous Diploblastic {Coelenterate) type to those enteroccelous Triploblastica, that have 

 afterwards developed into the Chordata {Urochorda, Hemichorda, Cephalochorda, and 

 Vertebrata). 



It will be seen that this statement excludes the idea of any direct ancestral relations 

 between Nemertea and Chordata. If any such relation were proposed, it might with 

 good reason be asked — considering the very extensive variation which is met with 

 amongst Nemertea — which species or which genus was more particularly pointed to. The 

 question in itself condemns the proposition which leads to it. 



It will, moreover, be seen that this statement accepts the outcome of Bateson's 

 researches and speculations, in so far as the points of agreement between Balanoglossus 

 and Amphioxus are fully recognised. A provisional link between these two, and an 



(ZOOL. CHALL. EXP. PABT LIV. — 1887.) HLh 16 



