164 THE VOYAGE OF H.M.S. CHALLENGER. 



Pentacrinoid ; while the apical dome plates in the Cyathocrinidse, 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 Palseocrinoids. 

 In the Blastoids, with the exception of Eleacrinus, they are generally small and irregular 

 as in CyathoGrinus ; but in the Platycrinidse, Actinocrinidse, and Rhodocrinidfe 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 Eleacrinus has 

 the deltoid piece on the anal side divided into two parts, which are separated by the anal 

 opening and its supporting plate. 



Professor AUman ^ pointed out long ago that many Palseocrinoids 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 Platycrinns and Actinocrinus which correspond 

 as I believe to the orocentral and five orals of Haplocrinus and Symhathocrinus, 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- 

 crinus, 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 in 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 Prebraehial stage in the development of Comatula, Trans. Roy. Sue. Edin., 1863, \-ol. xxiii. pp. 245-251. 



2 Amer. Journ. Sci. and Arts, vol. xiv. p. 119. 



