Echinoderms 59 



vary greatly in different forms. All have 

 a firm calcareous ' test " made of 

 numerous plates joined together by elastic 

 tissue, thus giving scope for enlargement 

 by the continuous adding of calcareous 

 matter to the edges of the plates. The 

 majority of forms are provided with 

 numerous spines, fitted to the " shell " 

 by ball and socket joints. These spines 

 are capable of moving in every direction, 

 and not only assist in carrying the urchin 

 from place to place, but also effectively 

 discourage the attacks of enemies. Be- 

 tween them are innumerable rows of 

 pincers, a few of which are provided with 

 poison glands and deal with small 

 creatures that may threaten the delicate 

 tissues between the plates. Certain of 

 these pincers, used for cleaning purposes, 

 have scythe-like blades that shave any 

 dirt from the skin, whilst some have very 

 fine " teeth/' which serve to hold minute 

 particles of food. The entire surface of 



