606 A. A. W. HUBRECHT. 



I would formulate the proposition^ to the further develop- 

 ment of which this memoir 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 character- 

 istic of those animal forms, by which a transition 

 has been gradually brought about from the archi- 

 ccelous Diploblastic (Coelenterate) type to those 

 enterocGjlous Triploblastica, that have afterwards 

 developed into the Chordata (Urochorda, Hemi- 

 chorda, 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 con- 

 demns 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 Am- 

 phioxus are fully recognised. A provisional link between 

 these two, and an arrangement of Balanoglossus as amongst 

 the Chordata, appears to be quite as justifiable as the elevation 

 of the Urochorda to their new dignity in zoological classifi- 

 cation. 



There is, however, a great diflFerence 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 type as one of transition in the way 

 above indicated. 



There is no doubt that the Nemertea represent a more primi- 

 tive phase than the Enteropneusta (Hemichorda). They have 

 no gill-slits ; but their nervous system shows certain unexpected 

 analogies with 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^ 



