Origin of (he Segmented Worms. oil 



but zoologists have not been able to agree as to the process 

 \vhicli best satisfies the conditions. But, on the other hand, 

 zoologists are all, I think, aureed that the form which comes 

 with tlio greatest probability next below the typical segmented 

 Worm is the Ccelenterate ; for as yet no satisfactory position 

 has been found lor the otluM- grouj)S of vermiform animals, 

 and though we usually place them near one another, we travel 

 outwards from the typical Annelid towards these grou|)S with 

 uncertain steps. AVe only reach solid ground again when we 

 con)e to the Coelenterates. Morphologists have, indeed, long 

 recognized that the Coclenterate body marks a well-defined 

 stage in the evolution of the Metazoa. The mechanism is 

 simple, yet very perfect, for the two primary functions of 

 animal life, the vegetative and the reproductive. Its success 

 may be gathered from the extraordinary wealth of the Coelen- 

 terates still peopling the waters of our planet both in numbers 

 and in forms. But we are justified in assuming that this 

 success has attended them from their first appearance in early 

 geological periods, bearing in mind that, as we go back to 

 simpler conditions of environment — simpler in the fact that 

 many animal forms preying and to be preyed upon were not 

 then evolved, — the variations in form would be less and less 

 numerous, until we reach a time when the simplest of all 

 conceivable types of Coelenterate was the only representative 

 of the race. 



It is to the Coelenterates therefore that we naturally look 

 first for the ancestors of the segmented Worms, and more 

 than one attempt has been made to sketch the form-changes 

 which would be necessary. So far as I remember, the earliest 

 and, to my mind, the best suggestion refers the segmented 

 Worms to the serial budding wliich is known to occur among 

 the Ccelenterates, e. g. in the " strobila." This is, however, 

 80 far only a suggestion ; I know of no attempt to work it 

 out in detail ; one apparently insuperable difficulty blocks the 

 way, viz. the presence, in the Annelids, of the coelomic 

 cavities, which are unknown in the Coelenterates. Prof. 

 Sedgwick* proposes to deduce the coelomic cavities of the 

 Annelids from the mesenterial chambers (^" gastric pouches ") 

 of a Ccelenterate-like ancestor. This would not explain the 

 addition of new segments, progressively diminishing in size, at 

 the posterior end. The method of the increase in tlie number 

 of the mesenteries which takes place in the higher Coelen- 

 terates is quile different. It seems to me that the reference of 



* Quart. Jcmrn. Micr. Sci. xxiv. (1884). 



34* 



