176 p. HERBERT CARPENTER. 



On the Apical and Oral Systems of the Echinodermata. 

 By P. Herbert Carpenter, M.A., Assistant Master at 

 Eton College. Part II. 



In a previous paper^ I have discussed at some length the 

 modifications of the Apical System of the Echinoderms in 

 the various members of the group. I now propose to treat 

 the Oral System in the same manner, commencing, as before, 

 with the Crinoids, in which order it reaches a very high 

 degree of development. 



After the disappearance of the ciliated bands encircling 

 the echinopaedium of Comatula, its anterior extremity be- 

 comes flattened and depressed in the centre. The raised 

 external rim of this depression exhibits a division into five 

 crescentic lobes, the oral lobes, which are situated interra- 

 dially. Opposite to the intervals between them are five 

 azygos tentacles marking the positions of the future radial 

 water-vessels, while interradially, opposite to each of the oral 

 lobes, is a pair of short tubular tentacles, borne like the 

 azygos one upon the water-vascular ring. 



The oral lobes are supported by the oral plates, which in 

 the earliest condition of the skeleton are situated directly 

 above, but in close juxtaposition to the basals. But as the 

 body increases in size, its equatorial portion, i.e. the band 

 between the upper edges of the basals and the lower edges 

 of the orals rapidly enlarges. During this enlargement the 

 oral plates maintain their original position, so that they be- 

 come completely separated from the basals by the growing 

 equatorial band, in the dorsal portion of which the radial 

 plates appear resting upon the basals. The orals are finally 

 left as a circle of five separate plates, each enclosed in its 

 lobe of perisome, on the centre of the new ventral or upper 

 surface of the larva, surrounding the mouth and enclosing 

 the circlet of ten interradial tentacles. This ventral surface 

 gradually widens as the radials enlarge and extend outwards, 

 carrying the perisome with them, to form the bases of the 

 arms. The " anal" plate appears in one of the interradial 

 spaces of the new formed disc, between the upper portions of 

 two of the first radial plates. The position of the anus indi- 

 cates that of the blastopore, which was primitively nearly in 

 the centre of the ventral surface of the Gastrula, and also 

 a point in the plane of division between the right and left 

 larval antimera ; for this plane is occupied by a circular 

 ' This Journal, Vol. XVIII, pp. 351-383. 



