ORAL AND APICAL SYSTEMS OF THE ECHINQDERMS. 203 



the calyx by the constant formation of new spines at the 

 base of each ray, which are supported by a long narrow 

 limestone plate, extending distinctly from the basal plate 

 almost to the terminal radial. This plate, according to 

 Agassiz, is also derived from the primitive abactinal system, 

 as are the superambulacral plate on the actinal surface with, 

 the subambulacral spurs formed from its edges. It is com- 

 parable to the antiambulacral arm skeleton of a Crinoid, 

 except that it is iiitra- instead of extra-radial. The super- 



FiG. XVIII.— Showing how the radials r of the young Starfish are carried 

 away from the basals (= genitals) b, and form the ocular plates at the 

 ends of the arms. 



ambulacral plate is absorbed in the Asterids, but persists in 

 the Ophiurids as the ventral plating of the arm. The 

 plating of the grooves on the ventral surface of the Crinoids, 

 although occupying a superambulacral position, does not 

 seem to have the same origin as the corresponding plating 

 in the Starfishes. The precise homologies of the ambulacral 

 grooves of the Crinoids in the other Echinoderms have yet 

 to be worked out, but I am inclined to think that Greeff's 

 suggestions' are right in their general principle. Agassiz^s 

 observations support them as far as the Urchins and Star- 

 fishes are concerned, while those of Gotte-, on the other hand, 

 demonstrate that the ambulacral grooves of the Crinoids are 

 a peripheral extension of the tentacular vestibule of the 

 larva, the floor of which forms the peristomial area whence 

 the groove-trunks radiate. This vestibule (fig. xiii, Ip) is 

 derived from the left or oral division of the enterocoel, so 

 that the ambulacral epithelium (fig. xiii, ae) covering its 

 floor and the so-called '^ ambulacral nerve" beneath it must 

 be hypoblastic in their origin. Further, in many Actmome- 

 trcB these structures are altogether absent from several of 

 the posterior arms, as if the growing vestibule had been un- 

 able to extend itself so far from the oral ring, which in this 

 genus is excentric or even marginal. 



We have yet to learn that the ambulacral or nerve-epithe- 

 lium of the other Echinoderms is a hypoblastic organ, and 

 that the grooves of the Starfish arms are derivatives of the 



1 "Ueber denBau der Echiuodermen," ' Dritte Mittheilun"-. Marburg 

 Sitzungsberichte,' No. 11, 1872, pp. 165—169. 



