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



wMcli they are well marked in tke extended parts. In the recent Crinoids the alternate 

 plates are represented by the ' Saumj)lattchen,' 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- 

 crinid^ 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 Actinocrinidse,^ 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 : — 

 "It 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 

 Actinocriuidse, 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 Gissocrinus 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 Cyatlwcrinus, 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 Waclismvith tells me that he Las lately found " tubular canals beneath the vault " both in Platijcrinus and in 

 some of the Rhodocrinidce (August, 1884). 



^ Revision, part ii. p. 29. ^ Ante, pp. 61-66. 



