524 



ZOOLOGY 



between the organs of all the series of parts resulted in the meta- 

 meric condition. 



Metamerism is not universal in the phylum. In some (Archi- 

 Annelida) it may be said to be incipient or rudimentary ; in others 

 (Gephyrea) vanishing or vestigial. The Archi-Annelida are in this, 

 as in some other respects, the most primitive of the Annulata, and 

 through them it seems possible to connect the higher members of 

 the phylum with such lower forms as Dinophilus (p. 337) and the 

 Histriobdellea (p. 338). The general occurrence of the trocho- 

 phore larva may be taken as pointing to descent from an unseg- 

 mented ancestor having resemblances to the trochophore, and a 



Fig. 418. — Diagram to illustrate possible relations of the unsegmented to the metamerically 

 segmented worm. A, unsegmented worm with differentiated head end ; B, pseudo-meta- 

 merism ; C, linear series of zooids in which the first zooid differs in character from the others, 

 and in which the formation of new zooids takes place at the posterior end ; D, metamerically 

 segmented worm. 



form like Dinophilus would afford us an intermediate link between 

 such a hypothetical ancestor and Polygordius or Protodrilus. 



The position of the unarmed Gephyrea in the Annulata is, as 

 already noticed, a matter of doubt ; if we dissociate them from 

 the Armata there is little to connect them positively with the 

 other members of the phylum. But, on the whole perhaps, the 

 evidence in favour of regarding them as allied to the Armata, 

 and through them with the Cheetopoda, is sufficiently strong. 



In adult structure, particularly in the absence of parapodia and 

 setae and the reduction of the coelome, the Hirudinea diverge some- 

 what widely from the Chsetopoda; but a study of their earlier 

 developmental stages shows unmistakably their close connection 

 with the latter group, more particularly with the Oligochseta ; and 

 the existence of an undoubted Leech (Acanthobdella) with setae 

 and with a well-developed ccelome traversed by mesenteries helps 

 still further to bridge over the gap between the two classes. 



