CRINOIDEA. 273 



Excepting for the anal interradius, which also contains 

 the primary water-pore of the larva, pentameral symmetry of 

 the Crinoid body is externally complete in almost all living 

 Crinoids.* In extinct Crinoids, however, it is frequently 

 disturbed by the insertion of the so-called anal plates in the 

 posterior (anal) interradius. (See p. 276.) 



As in most other Echinoderms the integument contains a large 

 amount of calcareous matter in the form of, for the most part, 

 regularly arranged plates, but movable spines t and pedicellariae 

 are conspicuous by their absence. 



The number of genera now living is comparatively small, but, 

 judged from the Crinoid standpoint, their variation in structure 

 is considerable ; indeed, in this respect they may be said to be 

 fairly representative of the immense number of forms which 

 lived in the Palaeozoic epoch, when the group attained its greatest 

 development. It is on account of these extinct forms that the 

 study of the skeletal parts has attained in this class that rela- 

 tively exaggerated importance which we found it to have in the 

 case of Echinoids. It is a subject of great difficulty and the 

 results arrived at, with regard to the structure of extinct forms, 

 are constantly undergoing modification. Moreover the struc- 

 ture of the skeleton varies in closely allied forms in char- 

 acters which it is customary to regard as having a high 

 morphological value. For instance in the Camerate families 

 Platycrinidae and Actinocrinidae the ambulacra of the calyx 

 cover may be exposed or subtegminal even within the limits of 

 the same genus (Wachsmuth and Springer), and the constitution 

 of the apical system of plates may vary in the most important 

 particulars in genera of the same family. 



Crinoids have indirectly had an important influence upon our 

 knowledge of the ocean. Until quite recently but two living 

 genera of fixed Crinoids were known, Pentacrinus and Holopus. 

 The discovery of Rhizocrinus in deep water by G. O. Sars in 

 1864, by the interest it excited on account of its stalked charac- 

 ter and general resemblance to extinct forms, led to the expedi- 

 tions of H.M.S. Porcupine, in 1868, and H.M.S. Lightning in 1869, 



would be as follows: right posterior radius = radius V, left posterior 

 = radius I, anterior radius = radius III, and right antero - lateral = 

 radius IV. 



: Thaurnatocrinus is the most striking exception, see p. 301. 

 f Spines have been described in one extinct genus (Arthrocanihd). 



Z III T 



