ORAL AND APICAL SYSTEMS OF THE ECHINODERMS. 179 



of the " tegmen calycis " is developed out of the narrow 

 equatorial zone of perisome intervening between the oral 

 and basal circlets of the young Pentacrinoid. It does not 

 seem to be present in the Palceocrinoidea, the solid vault 

 of which, as pointed out by Wachsmuth,^ is in no way homo- 

 logous with the ventral perisome of the recent Crinoids. 

 AUman^ has already pointed out that in many members of 

 this group there is a system of plates in the centre of the 

 vault, which is nothing but a more or less extensive de- 

 velopment of the simple oral system of the young Comatula. 

 Wachsmuth/' who has studied the complicated arrange 

 ment of these plates with much care, has somewhat un 

 fortunately termed them " apical," a name already applied 

 by previous writers to the plates which form the calyx at the 

 opposite pole of the body. In the simplest condition of these 

 so-called apical plates, as shown in genera which have but 

 a few vault pieces, there is a large central plate sur- 

 rounded by six others, viz. four large interradial ones of equal 

 size, and two smaller ones, also interradial, which are sepa- 

 rated by the anus, and together correspond to a single 

 larger one, Wachsmuth has found these " apical " plates 

 in the families Actinocrinida , Platycrinida, Rhodocriiiidce, 

 MelocrinidcR , and in a few isolated genera, including Cyatho- 

 crinus and Synhathocrinus, in which last they are repre- 

 sented by a number of small plates ; and he stales that 

 they " cover the central opening of the Blastoids, and can 

 be traced in many of the Cystideans."* They occupy a 

 central position in the vault, where there is much reason 

 to believe in the presence of a subtegminal mouth ; and they 

 are very largely developed in young specimens, from which, 

 as well as from other considerations, Wachsmuth infers 

 " that they were the first solid parts developed on the ventral 

 side in young Crinoids." 



Although the central plate was unknown to Allman^ 

 there can, I think, be little doubt that the six (practically 

 five) peripheral plates are comparable to the orals of the 

 young Comatula in the manner suggested by him. These 

 plates surround and support the walls of the tentacular 

 vestibule, into which thelarval mouth opens (fig.xiii Ip'), but 

 it does not acquire a communication with the exterior until 

 a comparatively late stage of development. Gotte's sugges- 



^ " Notes on the luternal and External Structure of Palaeozoic Crinoids," 

 'American Journal of Science and Arts,' vol. xiv, 1877, pp. 123, 190. 

 » Loc. cit., pp. 245-251. 



* Loc. cit., p. 187. 



* Loc. cit., p. 189. 



