270 



ECHINODERMATA. 



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

 rians. a, calcareous wheels of Chirodotu ; I, anchor with 

 supporting plate of Synapta ; c, chair-like bodies ; d, plates of 

 Holothuria impatient; e, hooks of Chirodotn. 



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

 drical form is obtained by an elongation of the axis (Holothuroidea] 

 (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 



a 



b of the inter-radii, 



the form takes 

 the shape of a 

 star (Aster oidea), 

 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 (Ophiuridw}, 

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

 jointed side twigs, the pinnuhe (Crinoidea). 



An important characteristic of the Echinodermata is the indura- 

 tion by calca- 

 reous deposits 

 of the deeper 



layers of the A 



integument 

 (dermal con- 

 nective tis- 

 sue), so as to 

 give rise to a 

 solid more or 

 less moveable 



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

 moveable ar- DR, dorsal marginal ossicles ; VR, ventral marginal ossicles ; Ap, 

 T +li ambulacra! ossicles ; Jp, intermediate interambulacral ossicles ; Atlp, 

 anterior adambulacral ossicles projecting into the mouth. 



leathery Holo- 



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 



