THE MORPHOLOGICAL COMPOSITION OF ANIMALS. 109 



evidence that there was a primitive creature of this general 

 character, of which the trochophores of Mollusca, Rot if era, 

 and Annelida are divergent modifications, and which was 

 unsegmented : the implication being that the segmentation 

 or the Annelida was superinduced. That this segmentation 

 resulted from gemmation is implied by what are called poly- 

 trochal larvee. These " sometimes appear as a stage succeed- 

 ing other larval types. Thus those of Arenicola marina arise 

 from larvae which at first were monotrochal, later became 

 telotrochal, and finally, by the appearance of new ciliated 

 rings between those already present, assumed the stage of 

 polytrochal larvag. . . . This condition warrants the 

 assumption that the segmented forms are to be looked upon 

 as the younger, the unsegmented, on the other hand, as the 

 phylogenetically older." (Korschelt and Heider, i, 278.) And 

 that the above-described rings of cilia mark off segments is 

 shown by the case of Ophryotrocha puerilis, which " remains, 

 as it were, in a larval condition, since the segments retain 

 their ciliation throughout life." {lb., 277.) Yet one more 

 significant fact must be named. In early stages of develop- 

 ment each segment of an archiannelidan has ccelomic spaces 

 separate from those of neighbouring segments, but in the 

 adult the septa "generally break down either partially or 

 completely, so that the perivisceral cavity becomes a con- 

 tinuous space from end to end of the animal." (Sedgwick, 

 Text Boole, 449.) While this fact is congruous with the 

 hypothesis here maintained, it is incongruous with the hypo- 

 thesis that the annelid was originally an elongated creature 

 which afterwards became segmented; since in that case the 

 implication would be that the ccelomic septa, not arising from 

 recapitulation of an ancestral structure, but originated by the 

 process of segmentation, were first superfluously formed and 

 then destroyed. 



Various lines of evidence thus converge to the conclusion 

 that an annulose animal is an aggregate of the third order. 



In June, 1865, when No. 14 of my serial containing the 



