36 STRUCTURE OF THE CYSTIDE^. 



appear to have been aware of the existence of special orifices in 

 the Crinoids for the passage of the ambulacral vessels, and hence 

 they still regarded the apical aperture as the mouth. Volborth says: 



"All the Cystideae were, like Crinoids, provided with articulated arms; and this 

 statement is not mere hypothesis, but is the result of philosophical induction from 

 distinct well-grounded facts, determined by observation — by the actual presence of arms 

 in some species, and the presence of tentacle furrows in others. The Cystideee were 

 also true Crinoids. Either in the young state or throughout life they were attached 

 by an articulated stalk, or by a pedicle, either to the bottom of the sea or to foreign 

 bodies. They had articulated arms which, as in Crinoidese, proceeded from the dorsal 

 pole of the cuticular skeleton. Diametrically opposite to the orifice of the pedicle is 

 placed the buccal orifice, and generally close to it is the sub-central anal orifice. 

 The cup differs however from that of the Crinoids, by such a predominance of the 

 dorsal side over the ventral, that the latter is often reduced to a minimum, consisting 

 only of the orifice of the mouth, so that the arms appear to be much nearer the mouth 

 than is the case with Crinoids." — Volborth on the Arms of CystidecB. Trans. Min. Soc. 

 of St. Petersburgh, 1845—6. 



Professor Miiller, whose extraordinary researches in the depart- 

 ment of the development of the recent Echinodermata have never 

 been excelled, says : — 



"The development of the antambulacral side of the radii in Crinoids takes place 

 either from the very base of the calyx, or from its circumference, or in the neighborhood 

 of the mouth, as in most Cystidea. In the latter case the calyx presents no radial 

 arrangement of plates from the base to the immediate neighborhood of the mouth ; it 

 begins only at the mouth in the oral arras, whose ambulacral grooves however lead to 

 the mouth, and, like the articulated antambulacral surface of the arms, present no 

 traces of the general plan of the Echinoderms. Hence it is intelligible why, so long as 

 the Cystideae were held to be armless, the radial arrangement of the Echinoderm was 

 unrecognized." — Uber den Bau der Echinodermen, p. 55. Translated by Mr. Huxley in 

 the Annals of Natural History, 2d series, vol. xiii. p. 242. April, 1854. 



Professor Miiller terms the dorsal side of the arm of a Crinoid 

 antambulacral, thereby distinguishing it from the ventral side, 

 whch is grooved and holds the ambulacra. The genital and ocular 

 plates of the sea-urchins are also antambulacral, but they are few 

 in number, and usually confined to one spot in the centre of the 

 back; and therefore the principles contained in the above extract 

 may be applied to shew that the Cystideas are not so nearly related 

 to the recent Crinoids as they are to the palaeozoic species. 



a. If we take a sea-urchin and placing it with the mouth upwards, 

 which is the usual position of that organ in the Crinoids, then 

 imagine a plane to be projected horizontally through the circular 

 area occupied by the antambulacral plates, the whole of the body 

 of the animal will lie above the plane. Its skeleton is composed 



