STRUCTURE OP THE CYSTIDE^. 35 



it must be exceedingly small. The same may be said of almost 

 every Cystidean that has been described and figured : the lateral 

 aperture is ever tte largest, and when it is considered that the one 

 in the apex, whether it be the mouth or not, always receives the 

 ambulacral vessels by which its capacity is further diminished, the 

 argument has still greater force. 



5. The objection that it is contrary to analogy that the mouth 

 of an Echinoderm should be situated any where else than in the 

 central point where the grooves meet, is not borne out by the facts, 

 because in a vast number of species of Crinoidea, the group most 

 nearly related to Cystidea, it is not so placed. The grooves do not 

 come near the mouth in the palaeozoic Crinoids, and therefore the 

 analogy wholly fails. 



6. With respect to the bearing of the arrangement of the 

 apertures of the recent Crinoids upon this question, there appears 

 at first sight to be some difficulty, as in these there are two 

 orifices, one of which not only receives the ambulacra, but is also 

 the mouth, while the other is said to be the anus. If we were 

 to be guided altogether by this analogy, the lateral opening of the 

 Cystideae would be, at least in some species, the anal aperture. 

 There is however a great difference in the structure of the two 

 groups. Any one who will take the trouble to examine, the figures 

 of the species given in most elementary works, will see that the 

 rays of Pentacrinus caput-Meduscs spring from the very base of the 

 body. The first radial plates of all the rays rest upon the upper, 

 joint of the column, and the others follow in succession up the 

 sides. If the point of the attachment of the column be the dorsal 

 side, then the rays are developed from the centre of the back. 

 The same structure occurs in Comatula. But in the Cystidea it is 

 the very reverse : the rays spring from the centre of the ventral 

 side. The whole of the cup of the Crinoid is radiated, but in the 

 Cystidean it presents no trace of a radial arrangement : all below 

 the apex is unradiated ; the bases of the arms are crowded together, 

 so that they all originate in a narrow space upon the upper side 

 of the body, where they surround the ambulacral orifice. The 

 first primary radial plates, which in Pentacrinus rest upon the 

 column, are in the Cystideae transferred to the opposite pole of 

 the body, and are there represented by the circle of plates which 

 surround the ambulacral orifice. This view of the structure of the 

 Cystideee was first put forth by Volborth, and afterwards adopted 

 by Professor J. Mtiller of Berlin. Neither of those authors however 



