A treatise on zoology . ce of the body, forming the biviumof the Holothurian, homologous with thebivium of the Pelmatozoa, as shown bythe position of the hydropore. The view has been held that theSynaptidae, with their simple structureand straight antcro-posterior gut, repre-sent the simplest and most ancestralEchinoderms. But if the above accountbe correct, this simplicity is only ap- r,ir^.nf inJ ivi fho iP«iilf of /Vn/fid«/a stngo of Svn<»;)/« (afterparent, anci is ino ICSUll OI It^ILSSlve somon). 7. ttMitaclos ; a/, nmdre- chanires. Such is the view that now porito;!u;
A treatise on zoology . ce of the body, forming the biviumof the Holothurian, homologous with thebivium of the Pelmatozoa, as shown bythe position of the hydropore. The view has been held that theSynaptidae, with their simple structureand straight antcro-posterior gut, repre-sent the simplest and most ancestralEchinoderms. But if the above accountbe correct, this simplicity is only ap- r,ir^.nf inJ ivi fho iP«iilf of /Vn/fid«/a stngo of Svn<»;)/« (afterparent, anci is ino ICSUll OI It^ILSSlve somon). 7. ttMitaclos ; a/, nmdre- chanires. Such is the view that now porito;!u;h: ,, . , J h, VoUnu lindB general favour. lllO I entaCtUUl ; Stage (iMg. XV 11.), in the (Uvelopnient t-aVoou-npicuies. x ac. of Sj/nupld, with five interradial circum- onil tentacles, slightly curved gut, and aboral anus, is therefore not the modern ontogenetic roprosontutive of the phylogonotic Pentadcea^ as Senion sup{) Fuj. XVII. ECHINODERMA—GENERAL DESCRIPTION 19 Further, if our present theory be correct, we must suppose thatthe larval history of the Holothurians has been exceedingly com-pressed ; so that, to take but one point, the development of thestraight larval gut into the coiled gut of the adult takes place, notby migration of the mouth and associated organs, but by lengthen-ing and twisting of the gut itself. It is noteworthy that the twolateral radii of the trivium with their nerves and muscles and tube-feet, as well as the oral tentacles to which they eventually give rise,develop much more slowly than the three other radii. Those arethe three radii which are assumed in the above account to behomologous with the original three radii of the primitive Pel-matozoan (cf. Fig. IX.). It therefore appears that the Holothurian stock branched offfrom the Pelmatozoa before complete pentamerous symmetry ofthe hydrocoel and associated organs had arisen, before any definitecalycinal system had develo
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