ORAL AND APICAL SYSTEMS OF THE ECHJNODERMS, 181 



Pentacrinoid stage of development, and ultimately opens 

 directly to the exterior. Wachsmuth's descriptions of some 

 natural casts of the structures below the vault of the 

 Actinocrifiida correspond in a most striking manner with 

 what Gotte has shown to be the condition of the young 

 Peiitacrinoid. 



He describes the centre of radiation for the concealed 

 ambulacra as a variously shaped space or plane, surrounded 

 by a furrow. The middle of this space is frequently occupied 

 by a small opening, or by a little cone indicating an aper- 

 ture leading to the inner cavity. This little central opening, 

 situated at the upper end of the vertical axis below the 

 vault, occupies, as Wachsmuth points out, the same position 

 as the mouth of Ajitedon occupies in the peristome (fig. xiii, 

 m). Now, this lip or peristome is nothing more than the 

 floor of the tentacular vestibule, which is closed till late in 

 the Pentacrinoid stage (fig. xiii, r), but ultimately opens to 

 the exterior ; while the corresponding space in Actmocrmus 

 remains permanently closed and covered in by the system of 

 actinal plates, namely, the single central one with the six 

 (=5) orals around it. 



Gotte's suggestion also helps us to understand why there 

 is no central actinal plate developed inside the oral circlet of 

 Comatula. For it would be situated precisely at the point 

 where the rupture of the peristome occurs that places the tenta- 

 cular vestibule, into which the true mouth opens, in communi- 

 cation with the exterior. Were it developed it would only be 

 in the way, and have to undergo resorption to a greater or less 

 extent, just as the central abactinal plate of many Urchins is 

 more or less completely resorbed after the appearance of the 

 anus. 



We find, then, that the oral system of the Palaocrinoidea 

 consists of a central actinal plate, around which six (= five) 

 oral plates are disposed. The resemblance between this 

 arrangement and that of the central abactinal plate, with the 

 five basals disposed interradially around it, strikes one at 

 once, and we thus find a complete correspondence between 

 the primitive conditions of the skeletal elements developed 

 around the two peritoneal sacs of the Crinoid larva. On the 

 actinal as well as on the abactinal side there is a single 

 central plate surrounded by five others, which are inter- 

 radial in position. There is, however, no known Crinoid in 

 which we find this primitive condition persisting, even as an 

 embryonic feature. For the central actinal plate is limited 

 to the Palaocrmoidea, while in all but Holojms (so far as is 

 known) the rudiments of the stem make their appearance at 



