XVI 



ECHINODERMATA 



513 



undergo, the characteristic oral disc of the young sea-urchin is built 

 up ; and the compound 

 structure composed of 

 these two organs is con- 

 veniently termed the 

 Echinus rudiment, 

 though it must never be 

 forgotten that most of the 

 body of the larva, and not 

 merely the Echinus rudi- 

 ment, is incorporated in 

 the body of the young sea- 

 urchin. 



The hydrocoele, seen 

 from the side, appears at 

 first like a circular disc ; 

 soon, however, a slight 

 notch appears in its pos- 

 terior border, the two ends 

 of this notch join, and so 

 the disc becomes a ring. 

 Before the ring form is 



am 



FIG. 388. Diagrammatic trans- 

 verse sections through the 

 "Echinus rudiment " of Echino- 

 plutei larvae of Echinus escu- 

 lentus ranging in age from 

 twenty-one to fifty days. 



A, Echinus rudiment just 

 formed by the apposition of the 

 amniotic cavity and the hydro- 

 coele. B, hydrocoele a ring ; 

 amniotic cavity closing. C, 

 amniotic cavity closed ; primary 

 tube feet of hydrocoele protrud- 

 ing into it epineural folds form- 

 ing. D, epineural folds united, 

 covering over epineural cavity 

 perihaemal rudiment formed. E, 

 amniotic roof burst, and primary 

 tube feet protruding perihaemal 

 rudiment developed into dental 

 sac and radial perihaemal canal, 

 urn, amniotic invagination and 

 cavity ; az.t, primary tube foot of 

 hydrocoelf ; ilent, rudiment of 

 tooth ; dent.s, dental sac ; ep, 

 epi neural cavity; e.p.f, epineural 

 fold ; hi/, hydroeoele ; hy.r, 

 hydrocoele rin.n ; '/''', Ifft pos- 

 terior coelom ; p.h, perihaemal 

 rudiment; /*./(.;, perihaemal radial 

 canal ; >:<:, radial canal of the 

 water-vascular system. 



VOL. I 



ax.t 



am 



r.c 



