5IO 



ECHINODERMATA — ECHINOIDEA 



urchin diverged widely (strong form of reaction to chemical 

 stimulus), exposing the gemmiform pedicellariae. These at once 

 seized the tube-feet of the enemy and the Starfish retreated, 

 wrenching off the heads of these pedicellariae ; then the Starfish 

 returned to the attack and the same result followed, and this 

 was repeated till all the pedicellariae were wrenched off, when 

 the Starfish enwrapped its helpless victim with its stomach. 



The minute trifoliate pedicellariae are brought into play by 

 any prolonged general irritation of the skin, such as bright light 



Fig. 226. — Views of a single blade of each kind of pedicellaria. A, blade of gemmiform 

 pedicellaria of Echinus elegans ; g, groove for duct of poison gland ; B, blade of 

 tridactyle pedicellaria of the same species ; C, blade of ophicephalous pedicellaria 

 of the same species ; r, ring for clamping this blade to the other blades ; D, blade 

 of trifoliate pedicellaria of £. alexandri. (After Mortensen.) 



or a rain of particles of grit or mud. They have the peculiarity 

 that not all the blades close at once, so that an object may be 

 held by two blades and smashed by the third. They may be 

 seen in action if a shower of powdered chalk is poured on the 

 animal, when they seize the particles and by breaking up any 

 incipient lumps reduce the whole to an impalpable powder, which 

 the cilia covering the skin speedily remove. In thus assisting 

 in the removal of mechanical " dirt " they earn the name which 

 Uexkiill has bestowed on them, of " cleaning pedicellariae." 



The ophicephalous pedicellariae, with their powerful bull-dog 

 grip, assist in holding small animals, such as Crustacea, till 

 the tube-feet can reach them and convey them to the mouth. 



