178 ECHINODERM ATA—PELMATOZOA SUB-KINGDOM III 
cylindrical or discoidal, and is composed of quadrangular, pentagonal, hexa- 
gonal, or polygonal plates, which are united by close suture. The plates 
vary in number from thirteen to several hundreds, and only exceptionally 
exhibit a regular arrangement. Sharp demarcations between the actinal and 
abactinal systems of plates, and between radial and interradial areas rarely 
exist ; the plates of the sides of the calyx pass insensibly into those of the 
ventral surface, and are disposed in regular cycles only in a few instances. 
The base, however, is composed of a distinct ring of plates, and is usually 
recognisable by the presence of an articular surface for the attachment of a 
stem, or by being directly adherent to some foreign object. 
The mouth is indicated by a central or subcentral aperture on the upper 
surface. It is sometimes covered by five small plates corresponding to the 
orals of Crinoids, and from it radiate from two to five simple or branching 
ambulacral grooves. The second opening on the ventral surface is situated 
eccentrically, and is frequently closed by a valvular pyramid, consisting usually 
of five or more triangular 
plates; or the covering 
may consist of a variable 
number of smaller pieces. 
This aperture, which was 
regarded by L. von Buch, 
Volborth, Forbes, and 
Hall as a genital open- 
ing, 1s now generally 
conceded to represent 
the anus. A third smaller 
opening, situated  be- 
tween the mouth and 
the anus, is present in a 
poe few forms only. The 
Fic. 292. Fic. 293. functions of this latter 

Glyptosphaerites Leuchtenbergi, Volborth. a, Aristocystites. Sub- orifice are not well un- 
Calyx showing ambulacral grooves, plated tegminal ambulacral Ue 
mouth-opening, large laterally situated anus, grooves; 6, Same of derstood, but it is com- 
and small ovarian aperture between mouth Pyrocystites. Enlarged ae 
and anus. (after Barrande). zs monly regal ded as the 
ovarian aperture, or genital 
pore (Fig. 292). Yet another small, slit-like opening, situated in the vicinity 
of the mouth, was detected by Barrande in the genus <Aristocystites ; but its 
functions are altogether unknown. 
The ambulacral grooves, which are present in most Cystideans, are usually 
simple, although sometimes distally branching, and are frequently roofed 
over by alternately arranged covering pieces. In a few forms (Caryocrinus, 
Cryptocrinus, ete.) the grooves are wholly absent. The genera <Aristocystites, 
Pyrocystites, and Craterina are without exposed ambulacral grooves; but they 
have instead, as Barrande discovered, a peculiar system of five or six covered 
passages on the inner surface of the calyx plates, which converge towards the 
mouth, and are distally more or less branching (Fig. 293). These structures, 
the so-called ‘‘hydrophores palmées,” were homologised by Barrande with the 
hydrospires of Blastoids ; but as Neumayr has pointed out, they are probably 
the equivalent of subtegminal food-grooves in Crinoids. 
The calyx plates exhibit most remarkable structural peculiarities. As a 
