38 STRUCTURE OF THE CYSTIDE^. 



compressed into so small a space that there is no longer any room 

 for the mouth in its centre. The organ is crowded outside of the 

 circle of radial plates, and lies lower down in the body. If the 

 arm-bearing plates of Caryocrinus ornatus — a species admitted to be 

 almost a Cystidean — were thus drawn together into the centre of 

 the ventral surface, the mouth would certainly be left outside, 

 because it is situated in the margin, between two of the arms. 

 It would not however on that account change its nature and 

 become a genital aperture. 



c. In all the species with an indefinite number of plates the 

 difference is still greater. If we extend the plane through the 

 base of the arms in E. aurantiim, we would have the very opposite 

 of what occurs in the instance of the sea-urchin. The body would 

 form a large globe-like expansion below the plane, while in the 

 sea-urchin it lies altogether above. The Cystideans of this type 

 must be regarded as the extreme in one direction, and the Echinidce 

 the extreme in another. Prof Miiller says: — "In an Echinoderm 

 which remains antambulacral quite up to the mouth, and develops 

 arms only from the oral part of the calyx, we have at its maximum 

 that condition which in the Echinid(B is at its minimum. To borrow 

 the phraseology of the ' Natur-philosophie,* we may say that the 

 «-aiyx of a PscudocrinUes, Agelocrinites^ Echinosph(Brites^ Echino- 

 rncrinites^ is the apex of an Echinus; it is however an expansion 

 of the apex large enough to enclose the whole of the intestines 

 of the animal, while in the Echinus these arc invested for the most 

 pait by the ambulacral zone of the perisoma." — TJher den Ban der 

 Echinodermen^ p. 16 ; Annals of Natural History, 2 ser. vol. xiii. p. 9. 



If now we place the above forms in series arranged according to 

 the greater or less concentration of the antambulacral side of the 

 rays towards the centre of the back, all the Crinoids would lie 

 between the Echinidae and the Cystideae. Thus : 



In the Echinidffi the antambulacral radiated skeleton is absorbed 

 into the middle of the back. 



In Pentacrinus and Comatula it is also strongly concentrated, but 

 covers the whole of the back and sides. 



In Thysanocrinus (Hall), a genus which appears in the Trenton 

 limestone, the unradiated space at the base of the cup is larger 

 than it is in Pentacrinus. There are two series of plates below the 

 first primary radials. There is then in this genus a tendency in the 

 radiated portion of the skeleton to depart from the centre of the 

 back and take its origin from the upper part of the body. 



