tlie Murplwlogy of the Echiiioderms. 113 



centre of the syzyglal cavities is separated from the coeliac 

 canal, which Prof. Perrier regards as their functional centre 

 of supply, by more than half the height of the arm-joint. 

 This is not the case in Antedon rosaceus^ the type chiefly 

 studied by Prof. Perrier; for it has relatively low arm-joints 

 with a deep ambulacra! groove on their ventral surface, so 

 that there is but a thin layer of limestone between the bottom 

 of the coeliac canal and the axial canal from which the syzy- 

 gial cavities radiate. But if Prof. Perrier had. had a more 

 extensive acquaintance with the different types of arm-joint 

 which occm* in the Comatidce and with the variations in the 

 sculpture on their syzygial faces, I cannot but think that he 

 would have hesitated before making the statement which has 

 been quoted above. 



It will be seen that he agrees with Dr. Carpenter in re- 

 garding these radiating syzygial furrows as nutritive in func- 

 tion, though he believes them to be filled with water from 

 the coeliac canal, rather than with the sarcodic basis-sub- 

 stance of the skeleton, which would maintain communication 

 between the internal and external tissues of the arm-joint, the 

 latter often reaching a considerable thickness. The origin 

 of these radiating canals in the central canal of the arm-joints 

 which lodges the neuro-vascular axial cord certainly agrees 

 better with the latter theory than with that of Prof. Perrier. 

 It maybe noted, too, that in his first account* of these cavities 

 in the Comatulce^ he said not a word about their communi- 

 cating with the exterior, as they sometimes seem to do in a 

 dried arm of Gomatula^ or in a fragment which has been 

 boiled in potash. He now tells us, however, that in the 

 Stalked Crinoids (Encrines) these radiating cavities are not 

 only present at the syzygies, but that they communicate with 

 the exterior by pores placed at equal distances round the out- 

 line of the syzygy. Can he name a single Stalked Crinoid 

 in which the syzygial faces are separated by radiating pas- 

 sages as in the Comatulce and there are pores round the out- 

 line of the syzygies ? Batliycrinus has no syzygies at all ; 

 and there are no pores or anything resembling them in 

 RMzocrinuSj Hyocrinus^ or Holopus. Prof. Perrier has never 

 seen a Metacrinus^ or he would scarcely have doubted its di- 

 stinctness from Pentacrinus'^ and, unless I am greatly mistaken, 

 he has never had an arm-fragment of the former genus from 

 which to cut a section through a syzygial union. The only 

 possible type, therefore, which could have furnished him with 

 the evidence on which he bases his statements respecting the 



* Zool. Anzeiger, 1885, p. 265. 



