STRUCTURE OF THE CYSTIDE^. 39 



When we examine a specimen of any species of Cyathocnnus, 

 we perceive another step in the same direction. There are, as in 

 the hist genus, two rows of plates below the radii ; but, in addition 

 to this character, the ventral side of the animal does not rise above 

 tlie upper margins of the first primary radials. The radiated 

 skeleton forms a belt round the upper part of the body, and 

 Cyalhocrinus is therefore nearer to Echino-encrinites than is Penia- 

 crinus, because the bases of the rays are nearer the ventral side. 



Many other examples might be given in illustration of this point, 

 were it necessary. The general conclusions I draw from the whole 

 of the above are, that when we compare the recent and ancient 

 Crinoids with the Cystidcce, the latter two are seen to be the most 

 nearly related upon the whole — that those genera with two series 

 plates below the radii are the nearest to the Cystideag, and that the 

 genera with the rays developed from the base are the most distant 

 of the Crinoids, the Echinidee being still further away. 



If these conclusions be correct, then the apertures of the Cystideae 

 should be more properly compared with those of the palaeozoic 

 than with those of the recent Crinoideae. These latter have no 

 special ambulacral orifice, the function being exercised through the 

 mouth. But the palaeozoic Crinoids have the ambulacral orifices 

 separate from the mouth, and so it would most probably be in the 

 Cystideae. 



Classification of the Cy slides. 



The following passage, extracted from the memoir of Professor 

 Miiller on the Structure of the Echinoderms, contains the outlines 

 of a system of classification which will most probably be generally 

 adopted. It should be borne in mind that he was not aware of the 

 existence of the ambulacral orifices in the Crinoids, otherwise no 

 doubt some portion of his paper would have been directed to their 

 examination : — 



'' Cystidece. — Among the Crinoids the CystidecB of L. von Buch form 

 a group which is distinguished by the inclusion of the genital organs, 

 together with the other organs, in the calyx. In the Pentacrinites 

 and Com.aiulcB on the other hand, the sexual organs are attached to the 

 pinnulae of the arms ; in those Crinoids which have only one calycine 

 opening (mouth), as Actinocrinus, Platycrinus, &c., the exclusion of 

 the sexual organs from the calyx is at once rendered probable by the 



