334 



ECHINODERMA. 



increase at the expense of the arms, absorbing these in its growth so 

 that they form merely the angles of a pentagonal disc (fig. 311). 

 In both arms and disc two surfaces are recognized, oral and 

 aboral, which pass into each other, usually without a sharp margin. 

 In the normal position the oral side is downwards and has in the 



FIG. 309. Comet form of Linckia multiflora. (From Korschelt-Heider.) One of the 

 arms is producing a new animal by budding. 



Fia. 310. 



FIG. 311. 



FlG. 310. Ophidiaster ehrenbergi. (After Haeckel). Comet form: one of the original 

 arms shown only in part. 



FIG. 311. Culcita pentangularis^ aboral view. (From Ludwig.) a, madreporite; &, re- 

 flexed end of ambulacral grooves. 



centre the mouth and radiating from it to the tips of the arms the 

 five ambulacral grooves. On the aboral surface is the anus (when 

 not degenerate) near the centre, and excentric from it in an inter- 

 radius is the madreporite (in many armed species two to sixteen 

 radii may have madreporites). 



A line passing through the madreporite and the opposite arm divides the 

 body into symmetrical halves. This ray is frequently spoken of as anterior, 

 since in the irregular sea urchins (Spatangoids) the homologous arm is 

 clearly anterior, while the madreporic interradius is posterior. This plane 

 of symmetry does not correspond with that of the larva. The two rays on 

 either side of the madreporite form the bivium, the three others the 

 trivium. 



The skin is everywhere protected by large and small plates 

 jointed together. These make a dry starfish hard and stiff, but 

 in life it is extremely flexible, the arms can be bent in any direc- 



