516 INVERTEBKATA CHAP. 



developed in the larva of E. miliaris. At the base of this aboral 

 pedicellaria a plate is developed. The other two pedicellariae are 

 situated on the right side of the larva, and are supported by a single 

 plate. A third plate begins to be formed around the madreporic 

 pore ; this is the beginning of the madreporite. All three plates 

 belong to the series of basals which we have already encountered in 

 the Asteroid and Ophiuroid, and which, as shown by Bury (1895), 

 form the genital plates of the adult. Small spicules, rudiments of 

 the remaining two plates, can now be detected. 



Whilst these changes are occurring, the right and left posterior 

 coeloms become dilated, approach one another, and fuse at the aboral 

 pole of the larva; and from the wall of the left posterior coelom, 

 where it abuts on the hinder wall of the left anterior coelom, a solid 



pro.b. 



FIG. 389. Diagrammatic side-views of larvae of an Asteroid, Echinoid, and Balanoglossid, 

 to show the comparability of the apical plate of an Asteroid with the larval brain 

 of the other two types of larvae. 



A, diagram of Bipinnaria larva, viewed from the side. B, diagram of Tornar'ia larva of Balanoglossid, 

 viewed from the side. C, diagram of Echinopluteus larva, viewed from the side, ap, apical plate ; l.br, 

 larval brain ; pr.o.b, prae-oral band of cilia ; pr.o.l, prae-oral loop of longitudinal ciliated band. 



bud of cells grows out and buries itself in the blastocoele which 

 intervenes between the stomach and the inner wall of the left 

 posterior coelom. This is the rudiment of the genital stolon. 



About the twenty-fourth day a remarkable larval nervous system 

 makes its appearance. It arises as a shallow ectodermal pit on the 

 dorsal aspect of the oral lobe, in the middle line behind the anterior 

 cross-bar of the longitudinal ciliated band. The ectodermal cells 

 lining this pit bud off from their bases a thick plexus of ganglion 

 cells and fibres. The floor of this pit may be termed the apical plate, 

 and compared with the apical plate of the Tornaria larva (Fig. 389). 



As the critical period of metamorphosis approaches, rudiments 

 of spines make their appearance on the three first -formed basal 

 plates. These differ from the spines already described, which are 

 formed from the floor of the amniotic cavity, in being quadrangular 

 in section with a crown of diverging points, and in being devoid 



