270 



ECHIXODEEMA.1^. 



flattened out to form a more or less extended surface. The cylin 



drical form is obtained by an elongation or 



*~ 



dothuroided) 



FIG. 210. Calcareous bodies from the internment of Holothu- 

 rians. a, calcareous wheels of Chirodota- b, anchor with 

 supporting plate of St/napta c, chair-like bodies ; d, plates of 

 Hulothuria impatims; e, hooks of Chirodota. 



(fig. 209), the round form by a shortening of the same and the penta- 



gonal disc by the latter process combined with a simultaneous elonga- 



tion of the radii. If the radii are elongated till they are two or more 



times the length 

 of the inter-radii, 

 the form takes 

 the shape of a 

 star (Asteroidea), 

 which may be 

 either flat or 

 arched. The arms 

 of the star may be 

 simple processes of 

 the disc, and en- 

 close a part of the 

 body cavity (Stel- 



leridea, Star-fish}, or they may be more independent moveable organs 



sharply marked off from the disc, and as a rule simple (Ophiuridat), 



but sometimes branched (Euryalidce), or they may even bear simple 



jointed side twigs, the pinnulce (Crinoidea). 



An important characteristic of the Echinodermata is the indura- 



tion by calca- 



reous deposits 



of the deeper 



layers of the 



integument 



(dermal con- 



nective tis- 



sue), so as to 



give rise to a 



solid more or 



less moveable 



or even im- 



moveable ar- 



mour. In the 



leathery IIolo- 



thuroidea (fig. 210) alone these skeletal structures are confined to 



isolated calcareous bodies, which are embedded in the integument, and 



have a definite form of latticed plates, wheels, or anchors. In these 



FIG. 211. Skeletal plates of Astropecten Hemprlchtii (after J. Miiller). 

 DIt, dorsal marginal ossicles ; VR, ventral marginal ossicles ; A/>, 

 ambulacral ossicles ; Jp, intermediate interambulacral ossiclea ; Ad]>, 

 anterior adambulacral ossicles projecting into the mouth. 



