XVI 



ECHINODEEMATA 



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 



Fig. 388. — Diagrammatic trans- 

 verse sections through the 

 " Echinus rudiment " of Eohino- 

 plutei larvae of Eohinus escu- 

 lentus ranging in age from 

 twenty-oce 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— 

 perihaeraal rudiment formed. E, 

 amniotic roof burst, and primary 

 tube feet protruding — perihaemal 

 rudiment developed into dental 

 sac and radial perihaemal canal. 

 am, amniotic invagination and 

 cavity ; as.t, primary tube foot of 

 hydrocoele ; dent, rudiment of 

 tooth ; dent.s, dental sac ; ej), 

 epineural cavity; ep.f, epineural 

 fold ; hy, hydrocoele ; hy.r, 

 hydrocoele ring; l.v.c, left pos- 

 terior coelom ; ^.\, perihaemal 

 rudiment; 2)./t.r, perihaemal radial 

 canal; r.c, radial canal of the 

 water-vascular system. 



VOL. I 



rrxrEaris 



