ECHINOIDEA. 225 



In some cases the tube-feet have been observed to transfer to 

 the mouth organisms held by the pedicellariae. The triphyllous 

 pedicellariae are supposed to be particularly concerned in keep- 

 ing the shell and spines clear of smaller particles such as sand 

 grains which may fall upon them. It is possible that all the 

 pedicellariae may assist in keeping the animals clear of foreign 

 organisms, whether animal or vegetable, which would naturally 

 attach themselves to the spines and shell (see, however, p. 196), 

 thus accounting for the clean condition in which the spiny shell 



FIG. 167. Blades of pedicellariae seen from the inner side ; 1 of a globiferous (Parechinus 

 miliaris), 2 of an ophicephalous (Strongylocentrotus droebachiensis), 3 of a triphyllous 

 (Parechinus miliaris), 4 of a tridentate pedicellaria (Strong, droebachiensis), a apophysis; 

 b basal part of bl blade ; et end-tooth ; st lateral tooth ; I articular surface (after Mortensen). 



is usually found. It is said that pedicellariae which have once 

 got hold cannot leave go, so that they must be torn off with the 

 bitten object. 



The spines are of various sizes and shapes. They are movably 

 articulated to tubercles on the shell plates. The larger tubercles 

 are called primary tubercles and carry the larger or primary 

 spines ; they consist of a mamelon which may be perforate or 

 imperforate according to the presence or absence of a ligament, 

 and of a boss which is the eminence supporting the mamelon ; 

 the scrobicule is the smooth area of the test round the boss. 



Z III Q 



