128 



ECHINODERMATA PELMATOZOA 



SUB-KINGDOM III 



body of the plates, enter only their outer angles. Other Fiatulata have a 

 madreporite. 



The ambulacra are frequently lined along their sides by perpendicular, 

 \\ edge-shaped side-plates, and these are surrounded by movable covering pieces, both 

 of which extend from the mouth to the ends of the arms. In the Palaeozoic 

 Taxocfinus (Fig. 228), the covering pieces are arranged in alternate rows, with 

 side 'pieces adjoining them. The latter plates occur also in most of the Fistula fa 

 and Articulata, but are rarely represented in the Camerata. The mouth may 

 be exposed or closed; being either surrounded by five oral plates (Taxocrinus, 

 Fig. 228), or the posterior oral may be pushed in between the four others, so 

 as to conceal the mouth ; the latter is then said to be subtegminal (Fig. 229). 



A very remarkable modification of the ventral disk occurs in the Palaeozoic 

 Camerata. Here the usually very numerous plates attain considerable thickness, 

 and fit into one another like the stones of an arch to form an extremely rigid, 

 more or less convex vault, which is sometimes surmounted by an equally rigid 

 plated proboscis. At the apex of the dome five large-sized plates are often 



FIG. 228. 



Taxocrinus intermed- 

 ius, W. and S. Ventral 

 disk (after Wachsmuth 

 and Springer.) 



PIG. :>_". >. 



Platycrinus Haiti, Shuni. Projection 

 of ventral disk, o, Anibulacrals ; in, In- 

 terambulacral areas ; in', Anal inter- 

 radius ; e, Covering pieces of the anibu- 

 lacrals ; i, Interradials ; p, Anterior 

 and lateral orals ; o, Posterior (anally 

 situated) oral ; x, Plates of the anal 

 interambulacral area (after W. and S.) 



Fie. 230. 



He.iucrtnus elonyatus, Goldf. Calyx 

 with tegmen. a, Profile; I, Aspect 

 from above. 



distinguishable, of which that lying in the anal interradius commonly differs 

 from the rest in form and size, and appears to be wedged in amongst the others. 

 These five plates are identified by Wachsmuth and Springer as orals. The 

 remainder of the tegminal plates are distinguished according to their position as 

 ambulacrals arid interambulacrals ; in most of the Actinocrinidae the ambulacrals 

 are not arranged in alternate rows (Fig. 231), but frequently consist of large 

 single plates of one or more orders, which are separated from one another by 

 the continuous interposition of supplementary pieces. In other groups, notably 

 the Platt/crmidae, the ambulacra are generally arranged in two rows of rather 

 large plates, which, however, lose their original character to some extent. The 

 interambulacrals usually meet with the interbrachials. The tegmen of the 

 Camerata, as a rule, is composed of large nodose plates, for the identification of 

 which considerable experience is required. 



Most of the Palaeozoic Crinoids have but a single opening in the tegmen, 

 which is interraclial in position, and undoubtedly represents the anus. With 

 the exception of the Ichthyocrinidae the mouth is subtegminal, and the food 

 grooves are rigidly closed. In many cases the covering pieces are pushed 



