332 



COMPARA TI I 'E ANA TOMY 



CHAP. 



in other Echinodermuta homologous with the interradials of the 

 Crinoidt'tt and the Blastoidea. In all endeavours to answer this 

 question the following plates should be kept in mind : in the 

 Ophiuroidea the interradially placed plates between the circle of radials 

 and the oral side (Fig. 287, p. 327), and among the Echiiioiilm, in 

 Tiarechwa* (Fig. 271, p. 319), the central of the three interradial plates 

 of an interambulacral area. 



Sub-Class 3. Cystidea. 



The spherical, pear-shaped, egg-shaped, or cup-shaped body of the 

 Cystidea is also enclosed in calcareous plates. In one of the principal 

 groups, that of the Eucystidea, the plating consists of numerous con- 

 tiguous plates arranged without any recognisable order. In this case 



-i/l? 



FIG. 204. System of plates of the apical capsule 

 of Caryocrinus ornatus, spread nut (after Hall). For 

 lettering see p. 317. 



FIG. 205. Cystoblastus Leuchten- 

 bergi, from the apical side. 11, Point 

 of insertion of the stem ; 8, anus ; 

 10, infra basals ; 12, pectinated rhonil'-.. 



a typical apical system of plates cannot be distinguished. In the 

 other principal group, the Cystocrinoidea, certain forms of which show 

 near relationship to the Crinuiilt'ii and Blastoidea, the test consists 

 of a relatively small number of plates, and a true apical system can 

 be found round the apical pole. 



The forms assumed by this apical system may be grouped around 

 two central types : Caryocrinus and Eehinoencrinus. The group 

 (.'((/t/ucriiiiix (Cwylocrinus, Hemicosmites, Juglandocfrinus) has its plates 

 arranged in six rays ; while the group Eehinoencrinus (Callocystis, 

 Lepadocrinus, Apiocystis, Cystolhistiis, Gh/ptocystis, Pleurocystis, Prim- 

 c//.sfa>, Pseudocnmis, etc.) shows the typical five-rayed arrangement of 

 the plates. In both groups the base is dicyclic, i.e. there is a circle 

 of infrabasals inside the circle of basals. 



Caryoerinus, six-rayed (Fig. 294). The circle of infrabasals 

 consists of four plates, two larger (which are contiguous) and two 

 smaller. Each of the two larger plates is double. Outside the circle 

 of the infrabasals lies a closed circle of six interradial basals, and 



