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, i.e. 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 bmchials (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 



i 





FIG. 223. 



l'i nt/ii'i'lnus civputmedusac, Lain. sp. 

 Ventral disk constructed of very thin 

 perisomic plates, with central mouth 

 (o), open ambulacra, and eccentric 

 anus (A). 



FIG. 224. 



Hyocrinus Bethellianus,"Wyv. Thorn. Recent. 

 Ventral disk, enlarged, o, Orals ; p, Mouth 

 (peristome) ; s, Covering plates; c, Dorsal 

 canals of the arms ; am, Ambulacral furrows of 

 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 calyds. The 

 covering may be in the form of a coriaceous skin, in which large numbers of 

 thin calcareous ossicles are embedded (Figs. 223, 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 



