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THE BOOK OF NATURE STUDY 



specimens with more or less rays, or with one large ray and the 

 beginnings of four small ones. These are forms which have 

 had their rays destroyed in some way, and which are in process 

 of re-growing them ; for it is characteristic of the Echinoderms 

 in general that they have an extraordinary power of regenerating 

 lost parts. In most of the starfishes it is parts which have been 

 accidentally lost that are re-grown in this way, but in the brittle- 

 stars the animal itself throws off parts of its own body, and then 

 unconcernedly proceeds to re-grow them. Turn over a stone in 



a pool, and you may 

 possibly see a wriggling 

 brittle-star beneath. Seize 

 it by one of the five arms, 

 and the arm will drop off, 

 the brittle-star making its 

 escape while you remain 

 with the actively moving 

 fragment. So prone to 

 self-mutilation indeed are 

 the brittle-stars that it is 

 often very difficult to ob- 

 tain a perfect specimen for 

 the aquarium. However 

 gently they are handled 

 they are always liable to throw off some part of the body. The 

 common brittle-star of pools (Ophiothrix fragilis) is the worst 

 offender in this respect, while the little sand-star (Amphiura elegans) 

 found under stones in the sandy pools, which does not mutilate 

 itself quite so readily, is to be preferred on this account for the 

 aquarium. In any brittle-star note the sharp distinction between 

 the arms and the disc as compared with starfish. The arms are 

 more slender and much more active, the animals moving by them 

 and not by their tube-feet. They consequently move more 

 rapidly and more actively than the starfish, and so make more 

 interesting inhabitants of the aquarium. The tube-feet of the 

 brittle-star are small, and are not placed in a groove as in the 

 starfish ; they occur at the sides and not on the under surface of 

 the arms. 



FIG. 77. Common brittle-star (Ophiothrix fragilis]. 

 sp, spines ; r, plates at base of arms. 



