178 



ECHINODERMATA— PELMATOZOA 



PHYLUM IV 



arranged in alternate rows, with side-pieces adjoining them. The latter plates 

 occur also in most of the Inadunata and Flexibilia, but are rarely represented 

 in the Camerata. The mouth may be exposed or closed ; either being 

 surrounded by five oral plates {Taxocrinus, Fig. 276), 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 suUegminal (Fig. 277). 



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

 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 

 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 and interambulacrals ; in most of the Batocrinidae the ambulacrals 



Fig. 276. 



Taxocrinus intermed- 

 ius W. and Sp. Ventral 

 disk (after Wachsmuth 

 and Springer). 



Fig. 277. 



Platycrinus Jialli Shum. Projection 

 of ventral disk. a, Ambulacrals ; ia, In- 

 terambulacral areas ; ia', Anal inter- 

 radius ; e, Covering pieces of the ambu- 

 lacrals : i, Interradials ; jj, Interior 

 and lateral orals ; o, Posterior (anally 

 situated) oral ; x, Plates of the anal 

 interambulacral area (after W. and Sp.). 



Fig. 278. 



Hexacrinus elongatus Goldf. Calyx 

 with tegmen. ■ a, Profile ; h, Viewed 

 from above. 



are not arranged in alternate rows (Fig. 279), 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 Platycrinidae, the ambulacrals 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 convex or nodose 

 plates, for the Identification of which considerable experience is required. 



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

 which is interradial in position, or sometimes central, and represents the anus. 

 With the exception of the Flexibilia the mouth is subtegminal, and the food 

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

 inward, and the ambulacra follow the inner floor of the tegmen, forming a 

 skeleton of ramifying tubes ; these are conducted along open galleries from 

 the mouth to the arm-openings (Fig. 280, A). 



2. The Arms (Brachia). — The arms of the Crinoid body form the immediate 

 Prolongation of the radials. The plates of the arms are termed hrachials, and 



