174 THE VOYAGE OF H.M.S. CHALLENGER. 
which they are well marked in the extended parts. In the recent Crinoids the alternate 
plates are represented by the ‘Saumplittchen, which, however, instead of forming 
a part of a solid vault, are movable, and line the lateral margins of the tentacle furrows.” 
Now these “Saumpliittchen” or covering plates of recent Crinoids are imbedded in 
the ventral perisome, and Wachsmuth admits that this is represented in the Actino- 
crinidee by the interpalmar areas on the upper surface of his internal casts, and in the 
calcareous network which lines the interior of the vault. He describes how the ambu- 
lacra pass outwards from the peristome within the body, and communicate directly with 
the arm-grooves. 
I believe myself that the tubular skeleton beneath the vault, which has thus far been 
observed only in the Actinocrinide,’ represents the covering of the disk-ambulacra of the 
recent Crinoids, passing at the arm-openings directly into the ambulacral skeleton of the 
arms. ‘The following passage” seems to imply that Wachsmuth is of the same opinion :— 
“Tt is now generally conceded that the tubular canals beneath the vault contain the same 
organs which in modern Crinoids are exposed on the ventral disk, and like them embrace the 
food passages and certain other vessels connected with the ambulacral system.” If then the 
tubular skeleton beneath the vault correspond to the covering plates of the disk in recent 
Crinoids, how can these last be represented by the alternating plates in the dome of the 
Actinocrinide, which, as Wachsmuth himself admits, are not readily distinguishable ? 
Another difficulty also presents itself in connection with Wachsmuth’s views respect- 
ing these alternating plates of the ambulacra. Those on the calyx he considers as vault 
pieces ; those on the arms as representing the covering plates of recent Crinoids. But in 
another place he tries to prove that they are rudimentary pinnules, a question which has 
been already discussed. * 
Now it is obvious that the plates covering one end of an ambulacrum cannot be vault 
pieces, while those at the other end are covering plates or rudimentary pinnules—they 
cannot be both. It appears to me tolerably certain that the whole series of regular 
alternating plates, calicular and brachial, represent the covering plates on the disk and 
arms of recent Crinoids; but I will not venture to assert that they were invariably 
movable on the vault and free rays, so as to expose the food-grooves to the exterior, 
The condition of Gissoerinus seems to me to confirm this view very strongly, and 
also to emphasise the difference between Cyathocrinus and Coccocrinus, to which I have 
alluded above. 
The composition of the calyx in Gissocrinus is the same as in Cyathocrinus, but the 
lowest arm-joints instead of resting on the outer edges of the radials, lie upon their 
ventral surface, and extend downwards towards the peristome over the sutures between 
1 Mr Wachsmuth tells me that he has lately found “tubular canals beneath the vault” both in Platycrinus and in 
some of the Rhodocrinide (August, 1884). 
? Revision, part ii. p. 29. 3 Ante, pp. 61-66. 
