482 



ZOOLOGY 



SECT. 



becoming each complete in itself, not, under ordinary circum- 

 stances, becoming detached. The establishment of a closer 

 connection between the various organs of such a colony with the 

 special differentiation of the anterior end would result in a con- 

 dition closely resembling the metamerism of the Annulata. It is 

 conceivable that a condition of pseudo-metamerism was followed 

 by that of a linear series, not of zooids, but of comparatively 

 independent parts capable of readily reproducing the animal 

 when detached by accidental injury, and that a secondary closer 

 connection established between the organs of all the series of parts 

 resulted in the metameric condition. 



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

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



FIG. 384. Diagram to illustrate possible relations of the unsegmented to the metamerlcally 

 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, metanierically 

 segmented worm. 



(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. 310). The 

 general occurrence of the trochosphere larva may be taken as 

 pointing to descent from an unsegmented ancestor having re- 

 semblances to the trochosphere, and a form like Dinophilus would 

 afford us an intermediate link between such a hypothetical ances- 

 tor 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 Chaetopoda, is sufficiently strong. 



