METHODS OF ATTACK AND DEFENCE 89 



and " ophicephalous," the functions and reactions of which 

 have been studied by von Uexkiill (1899). Each pedicellaria 

 includes a clawed portion or head with an independent 

 nervous centre controlling the opening and closing of the 

 claws or blades. The " head " is supported on a long stalk, 

 also innervated, and rigid or flexible as the case may be. 

 Two only of the four types concern us here : the tridactyle 

 and the trifoliate pedicellariae. These have been shown by 

 von Uexkiill to be concerned with the removal of foreign 

 material from the " test." The tridactyle form is large and 

 conspicuous with three pointed blades each with a double 

 row of teeth along its edge, borne on a slender stalk. The 

 blades, which close very accurately, are worked by dis- 

 tinct sets of adductor and abductor muscles. A very slight 

 mechanical stimulus is sufficient to throw the pedicellaria 

 into action and to cause them to grasp and destroy foreign 

 organisms such as the minute swimming larvae of various 

 sessile parasitic animals which would otherwise settle on 

 the delicate exposed ectoderm of the sea-urchin. In con- 

 sequence Uexkiill calls them " snap-pedicellariae." The 

 trifoliate pedicellariae are of much smaller size. Their 

 function is similar but their method of action affords an 

 interesting contrast. Any prolonged general irritation of the 

 skin, such as bright Hght or a rain of particles of grit or 

 mud, will bring them into action, and they have the pecu- 

 liarity that the blades do not close simultaneously, so that 

 an object may be held by two blades and smashed by the 

 third. Their function may be studied by causing a shower 

 of ground chalk to fall on the animal, when the particles are 

 seized and broken down to an excessively fine powder which 

 the ciHa covering the skin speedily remove. In view of their 

 function of removing foreign particles Uexkiill has given 

 them the name of " cleaning pedicellariae." 



It is evident that Echinoderms, particularly sea-urchins, 

 with their sluggish habits and broad, firm surface, are particu- 

 larly liable to being settled upon by encrusting organisms. 

 To this serious menace the high degree of specialisation of 

 the cleansing organs is evidently to be ascribed. In this 



