842 EMBRYOLOGY 



tlie ontogenetic recapitulation of an ancestral form which 

 was common to the Annelida, Mollusca, and Molluscoidea, 

 and from which these animal stems branched off as inde- 

 pendent groups. The assumption that the Trochophore cor- 

 responds to an ancestral foi-m is supported, not alone by 

 the circumstance that the lai'val forms in the groups men- 

 tioned can all of them be traced more or less directly to 

 the Trochophore : it acquires a further and important sup- 

 port from the fact that in the division liotifera we see before 

 us forms which in their adult condition remain essentially 

 at the stage of organization of the nnsegmented Trocho- 

 phore. We have already mentioned (p. 259) that not only 

 the Rotifer known by the generic name of Trochosphaera, 

 but also the rest of the Rotatoria, can readily be referred to 

 the plan of the Trochophore. The Rotatoria accordingly 

 are organisms which still exhibit the closest relationships to 

 the Trochophore-like ancestral form whose mode of locomo- 

 tion and plan of organization, with some secondary changes, 

 they have retained. 



If we take into comparison the rest of the groups of the 

 so-called Vermes, there is apparent, in the first place, a 

 striking resemblance between the Trochophore of the An- 

 nelida and the larval form of the Nemertini known as 

 Pilidium (comp. p. 231), even though in their further de- 

 velopment the two groups pursue different paths. By means 

 of the Pilidium we are also led to bring certain larvas of the 

 Tnrhellaria into remote comparison with the Trochophore 

 (comp. pp. 168 and 230). 



In searching for the ancestral forms from which the 

 Trochophore-like archetype arose one meets with great diffi- 

 culties. In order to ari-ive at an idea of this ancestral form, 

 the Trochophore has been compared to a Medusa. Its 

 pelagic mode of life, its shape, and, above all, the nerve- 

 ring of the ciliated band discovered by Kleinenberg, were 

 the things which led this author and Balfour to assume its 

 descent from a medusoid form. Derived in such a way, the 

 preoral band of cilia is, from its position, referred to the 

 margin of the umbrella, and the aboral dome of the Trocho- 

 phore to the ex-umbrella, whereas the part of the larva lying 



