146 ECHINODERMATA— PELMATOZOA phylüm iv 



exhibit a regulär arrangement. Sharp demarcations between the acfcinal 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 regulär 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, or at the end opposite to that which is attached to the column or 

 stem. 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 valvulär pyramid, consisting usually 

 of five or more triangulär 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 opening, is now generally con- 

 ceded to represent the anus. A third smaller opening, situated between the 

 mouth and the anus, is present in a few forms only. The functions of this 

 latter orifice are not well understood, but it is commonly regarded as the 

 ovarian aperture, or genital pore (Fig. 228). 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, or food-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 Calix 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. 

 229). These structures, the so-called " hydrophores palmdes," 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 or thecal plates exhibit most remarkable structural peculiarities. 

 As a rule they are more or less extensively perforated by pores or fissures ; 

 although in some forms [Cryptocrinus, Malocystites, Ateleocystites, etc.) they 

 appear to be imperforate, and are composed of a homogeneous calcareous 

 layer of greater or less thickness, the same as in Crinoids. But in Aristo- 

 cystites, Calix, Proteocystites, Glyptosphaerites, Echinosphaerites, etc., the plates 

 are uniformly covered both externally and internally with a very thin, 

 generally smooth, calcareous membrane, which may be perforate or imperforate. 

 The central layer is of varia,ble thickness, and is traversed by numerous 

 canals (Figs. 229, 230) which extend from the inner to the outer surface, 

 sometimes rectilinearly {Aristocystites, Calix, etc.) ; sometimes in slightly 

 sinuous lines ; and in rare instances they divide dichotomously. The canals 

 terminate on either surface in small round apertures or pores, which are 

 arranged either singly or in pairs, and may or may not penetrate the outer 

 calcareous membrane. The pores are commonly situated either on a tubercular 

 elevation, or in a slight superficial depression. 



But still more frequent than the canals are the so-called pore-rhomhs (Fig. 



