164 THE VOYAGE OF H.M.S8. CHALLENGER. 
Pentacrinoid ; while the apical dome plates in the Cyathoerinidee, to which reference has 
been made already, as covering the central opening of the summit, do not reach anything 
like either the size or the regularity of arrangement that is to be found in the plates 
which have been described by Wachsmuth under the same name in other Paleeocrinoids. 
In the Blastoids, with the exception of Eleacrinus, they are generally small and irregular 
as in Cyathocrinus; but in the Platycrinide, Actinocrinidee, and Rhodocrinide they form 
a group of seven plates in the centre of the vault. The central plate is surrounded by 
six others, four of which are of equal size, while the remaining two are smaller. They 
are separated by the anal tube, and correspond to a single plate, just as Hleacrinus has 
the deltoid piece on the anal side divided into two parts, which are separated by the anal 
opening and its supporting plate. 
Professor Allman’ pointed out long ago that many Paleocrinoids have a group of 
plates in the centre of the vault, which is nothing but a more or less extensive develop- 
ment of the simple oral system of the young Comatula. This pregnant suggestion refers 
to the seven apical dome plates of Platycrinus and <Actinocrinus which correspond 
as I believe to the orocentral and five orals of Haplocrinus and Symbathocrinus, having 
been developed like them on the left larval antimer; while the rest of the vault in 
Actinocrinus is a further extension of this oral system, which is unrepresented in the 
Neocrinoids. Thus then, though Platycrinus and Actinocrinus are in the condition of 
having the tentacular vestibule and peristome permanently closed, just as in Haplo- 
erinus, yet they have undergone an immense development upon this condition as a basis. 
The tentacular vestibule in the Pentacrinoid larva is merely the peristome concealed 
beneath the oral pyramid ; but in Actinocrinus it is greatly enlarged so as to take in the 
whole surface of the disk; and the ambulacra passed over this surface towards the 
central mouth from the periphery of the disk, where they entered the dome from the 
arms through the well known arm-openings or ambulacral openings. These gave passage 
not only to the ambulacra proper or food-grooves, but also to extensions of the body- 
cavity, and to the radiating trunks of the nervous, blood-vascular, and water-vascular 
systems. All these last lay between the body-cavity and the food-grooves, and converged 
towards their respective circum-oral centres. The upper surface of internal casts of the 
vault of Actinocrinus is marked by bifurcating ridges which indicate the position of the 
food-grooves radiating from a central peristome, just as im the disk of a recent Antedon, 
as has been pointed out by Wachsmuth.’ Traces of these ambulacra are often found in 
the interior of the vault. In many cases they were covered in by a double row of 
alternating plates just like those of the arms, with which they were continuous at the 
arm-openings. They were floored by a double row of plates, and so formed tunnels beneath 
the vault, but closed independently of it by the covering plates on their upper surface. 
1 On a Prebrachial stage in the development of Comatula, Trans. Roy. Soc. Edin., 1863, vol. xxiii. pp. 245-251. 
2 Amer. Journ. Sct. and Arts, vol. xiv. p. 119. 
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