126 ECHINODERMATA—PELMATOZOA SUB-KINGDOM III 
The upper boundary of the calyx is differently demarcated by different 
authors. Many assign all the plates above the first cycle of plates in each ray 
to the arms, even when they are immovably united with one another at the 
sides ; while, according to Schultze and others, the arms begin invariably at the 
point where they first become movable, 2.c. above the first articular facet. The 
latter course is open to serious objections, inasmuch as strictly homologous 
parts receive different appellations in different groups. 
Carpenter, Wachsmuth, and Bather restrict the term “radial” to the lower- 
most circlet of radially situated plates, and speak of the succeeding cycles as 
far as and including the first axillary plate as brachials (distinguished as first, 
second, and third costals, distichals, and palmars respectively), in all cases, 
whether the plates are free or fixed. 
In most Palaeozoic Crinoids one or more interradial plates are intercalated 
between two of the rays, and in line with the anal aperture; these are called 
the anal plates or anals. If a plane be passed through the latter and through 
the radial situated directly opposite, the calyx will be divided into two 
symmetrical halves: the parts lying to the right or left when viewed from the 

Pic. 223. Fic. 224. 
Pentacrinus caputmedusae, Lam. sp. Hyocrinus Bethellianus,Wyv. Thom. Recent. 
Ventral disk constructed of very thin Ventral disk, enlarged. 0, Orals; p, Mouth 
perisomie plates, with central mouth (peristome); s, Covering piates; c, Dorsal 
(0), open ambulacra, and eccentric canals of the arms; am, Ambulacral furrows of 
anus (4). the arms ; an, Anus (after Wyville Thomson). 
posterior or anal side are so designated ; while the anterior side is that opposite 
the anal interray. Interradial plates, however, are not confined to the anal 
interray, but are frequently developed also between the other rays, when the 
calyx is correspondingly expanded. If several cycles of radials are present, an 
equal number of interradials are also developed, and are distinguished in like 
manner as interradials and distichal interradials of various orders. The anal 
interray is frequently characterised by the peculiar number, size, and position 
of the anal plates. 
c. The superior side of the calyx is known as the tegmen calycis. The 
covering may be in the form of a coriaceous skin, in which large numbers of 
thin calcareous ossicles are embedded (Figs. 225, 224), or of a plated disk 
rising from the base of the arms. It frequently exhibits a more or less central, 
externally visible mouth-opening, and a usually eccentric interradial anal 
aperture. The mouth opens into an oesophagus and thence into the expanded 
visceral mass, which fills the greater portion of the inner cavity. The intestinal 
canal is directed downwards at first, and after numerous windings discharges 
into the anal opening. In certain fossil Crinoids (Actinocrinidae) the digestive 
