334 



COMPARATIVE ANATOMY 



CHAP. 



roof of the vestibule. This roof is formed of five interradial lobes, 

 supported by five interradial skeletal plates, the oral plates. An aper- 

 ture only arises secondarily at the apex of the roof, and the five oral 

 lobes separate in such a manner that the tentacles can project through 

 the clefts between them. The mouth is now in open communication 

 with the exterior. 



At first the five oral plates rest directly on the oral edges of the 

 basal plates of the apical system. But in proportion as the calyx 

 increases in size, and the arms grow out, the distance between the 

 basals and the newly-formed radials, which support the arms, on the 

 one hand, and the oral plates on the other, becomes greater and greater, 

 since the latter remain at the centre of the tegmen calycis, surround- 

 ing the mouth. There thus arises, between the bases of the arms and 

 the circle of the oral plates, which in comparison with the continually 



growing calyx be- 

 comes more and more 

 insignificant, a cir- 

 cular zone, the peri- 

 pheral zone of the 

 tegmen calycis. The 

 food grooves running 

 out from the mouth, 

 passing between the 

 five oral lobes, tra- 

 verse this peripheral 

 zone of the tegmen 

 to the bases of the 



FHJ. >7. Haplocrinus mespilifonnis (atu-r Wachsmuth and arms. This peri- 



Springer). A From the anal side ; B ; from the oral side 1, Orals; heral Z(me CQntinu . 



2, oral pole ; 3, anus ; 4, radials ; .-, inferradial ; 0, basals ; ( , first L . . 



brachial; 8, point of attachment of the arm. ally increases 111 S1Z6, 



while the central part, 



surrounded by the five oral lobes, does not grow further, and forms 

 an ever-diminishing central region of the tegmen calycis. Finally, the 

 oral plates, with the lobes, are entirely resorbed, and the minute 

 central zone can no more be distinguished ; the whole oral surface of 

 the Antedon calyx is a free disc, by far the greater part of which has 

 been formed outside the base of the oral pyramid. In the centre of 

 this oral disc the mouth lies uncovered, and on the surface of the 

 disc the food grooves are visible running out radially to the bases 

 of the arms. 



Among the immense array of forms comprised under the crinoids 

 we find a few groups with five oral plates forming, as in the larva of 

 Antedon, the whole skeleton of the tegmen calycis. In the Inadunata 

 larviformia, type Haplocrinus (Fig. 297), there is actually a closed 

 pyramid of five oral plates, which, at the edge of the calyx, rest on 

 the radials of the dorsal cup. Only at the bases of the arms do 

 the five oral plates separate to form five radial apertures, through 



