178 



ECHINODERMATA 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, is now generally 

 conceded to represent 

 the anus. A third smaller 

 opening, situated be- 

 tween the mouth and 

 the anus, is present in a 

 few forms only. The 

 functions of this latter 

 a, Aristocystites. Sub- orifice are not well un- 



tegminal ambulacral ^ . . 



rooves; &, Same of derstOOQ, but it IS COm- 



llarged monly regarded 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, etc.) 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 palme'es" 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 



FIG. 292 



Glyptosphaerites Leuchtenbergi, Volborth. 

 Calyx showing ambulacral grooves, plated 

 mouth-opening, large laterally situated anus, 

 and small ovarian aperture between mouth 

 and anus. 



Pyrocystites. 

 (after Barrande). 



