3LASS III CRINOIDEA 175 



ery numerous short fibres of connective tissue, which may become more or 

 ess calcified ; this forms an immovable Union of the type known as a close 

 uture, in which the plates are immovably held together by fibrous con- 



ective tissue. Though usually smooth, the Joint faces are sometimes 

 triated, which striations are visible externally as incised lines. 



b. Succeeding the base is a cycle of five (rarely four, six, or ten) plates, 

 which, on account of their position with reference to the rays, are called 

 radials. The radials form the sides (more rarely the floor) of the calyx 

 in nearly all Mesozoic and Recent Crinoids, and give origin directly to 

 the arms, which may become free immediately above the radials, or may be 

 incorporated for some distance in the calyx, either by means of supplementary 

 plates, or by lateral union among themselves. 



The Upper boundary of the calyx is differently demarcated by dilFerent 

 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 became 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 Springer, and Bather restrict the term " radial " 

 to the lowermost circlet of radially situated plates, and consider the succeed- 

 ing cycles as far as and including the first axiliary plate as hrachials (dis- 

 tinguished as first, second, and third costals, distichals, and palmars ; or as 

 first, second and third primibrachs (IBr), secundibrachs (IIBr), and terti- 

 brachs (IIIBr) respectively), in all cases, whether the plates are free or 

 fixed. 



In most Paleozoic 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 

 posterior or anal side are so designated ; while the anterior side is that oppo- 

 site 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 and 

 brachials are present, an equal number of interradials may be 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. 271, 272), 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 ex- 

 panded visceral mass, which fills the greater portion of the inner cavity. The 

 intestinal canal is directed downwards at first, and after making usually one 

 complete circle, more rarely after numerous windings, discharges into the 

 anal opening. In certain fossil Crinoids (Actinocrinidae) the digestive appara- 

 tus is represented by an extremely thin-walled, finely perforated, convoluted 



