PHYLUM ANNULATA 



511 



during this process serially repeated outgrowths of the enteron 

 became separated off to form series of coelomic sacs enclosing the 

 gonads, a condition would be reached not far removed from that 

 which characterises the Annulata. On the other hand, the meta- 

 meric condition is deducible from the condition of a linear colony 

 of zooids proliferating at the posterior end, the zooids, though 

 becoming each complete in itself, not, under ordinary circumstances, 

 becoming detached. The establishment of a closer connection 

 between the corresponding organs of the zooids in such a colony, 

 with the special differentiation of the anterior end, would result in 

 a condition 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, and that a secondary closer connection established 



Fia. 425. 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. 



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

 meric condition. 



It has to be borne in mind, however, in estimating the breadth 

 of the gap between the Annulata and the Platyhelminth that the 

 former differs from the latter not only in the presence of meta- 

 meric segmentation, but also in the possession of a totally distinct 

 type of nervous system. In this respect, in fact, Gordius (Nemat- 

 helminthes) is apparently nearer the Annulata than are any of the 

 Flat- worms. 



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

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

 (Echiurida and Sipunculoidea) 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 



