18 



ECHINODERMA GENERAL DESCRIPTION 



has gone the genital pore ; and the madreporite, though uncon- 

 nected with the ex- 

 terior in the adult of 

 most now living, must 

 have moved towards 

 this end also. But 

 there is an important 

 difference between this 

 and the Dipleurula in 

 the coil of the gut, and 

 Fl - XVI - the vastly altered rela- 



Diagrammatic reconstruction of the imagined primitive fi r v na n f fV, prplrrnip 



Holothurian type, for comparison with Figs. I., VII., and tlum 



XI. 0, mouth ; As, anus ; M, hydropore ; l.hc, left hydro- cavities with their in- 

 coel ; g, genital opening. 



tervemng mesenteries. 



The arrangement indicates that the mouth, hydrocoel, madre- 

 porite, and associated organs of a Pentactcea gradually moved 

 anteriorly away from the anus, thus coming nearer to the 

 stem (preoral lobe of larva) and lengthening the gut by another 

 half-coil. When the fixed existence was given up and the 

 food -grooves closed in, leaving the external podia from the 

 water-vessels, then the rays were able to extend equally in all 

 directions from mouth to anus. The anterior ray and the two 

 adjoining rays had thus cpme to be on that side of the elongate 

 body which was directed towards the sea-floor, and to this they 

 clung, or on it they crawled, by suckers 

 which developed on the podia, which 

 thus became " tube-feet." The left and 

 right posterior rays ran along the upper 

 surface of the body, forming the bivium Stc 

 of the Holothurian, homologous with the 

 bivium of the Pelmatozoa, as shown by 

 the position of the hydropore. 



The view has been held that the 

 Synaptidae, with their simple structure 

 and straight antero-posterior gut, repre- 

 sent the simplest and most ancestral 

 Echinoderms. But if the above account 

 be correct, this simplicity is only ap- 

 parent, and is the result of regressive Semon) T< tentacle8 . ^ , nadre . 



changes. Such is the view that now porite ; stc, stone canal ; st, stomach ; 

 , . . mi n . , 7 oc, supposed otocysts ; pb, Polian 



finds general favour. The Pentactula vesicle depending from hydrocircus; 



stage (Fig. XVII.), in the development i^.^Si'TS" **' " 

 of Synapta, with five interradial circum- 



oral tentacles, slightly curved gut, and aboral anus, is therefore 

 not the modern ontogenetic representative of the phylogenetic 

 Pentactcea, as Semon supposed. 



