SEA-URCHINS, STARFISH, AND BRITTLE-STARS. 



129 



ducing the appearance of a little brush. At the sides of 

 the short rays there are prominent lateral spines of simple 

 structure. There can be no difficulty in recognising the 

 sun-star, but it is interesting to notice how it differs from 

 another species, Solaster endeca, which is sometimes flung 

 on the beach by storms. This has nine to eleven arms, is 

 usually purple, not purplish red, has more numerous dorsal 

 spines more closely packed together, and less distinct lateral 

 spines. 



After the starfishes we come to the Ophiuroids, or brittle- 

 stars, which from their shape and habits are perhaps less 

 conspicuous than 

 the starfishes, but 

 are quite as abund- 

 ant. They are to 

 be found under 

 stones or among 

 weed, twining 

 their, long snaky 

 arms about the 

 surrounding ob- 

 jects, and snap- 

 ping them off at 

 a touch. At least 

 three species are 

 common in the 

 living condition 

 between tide- 

 marks, and others 

 occur at times after storms. Instead of giving a formal 

 definition of the group, let us look at the general charac- 

 ters of our common brittle-star (Ophiothrix fragilis, see 

 Fig. 42). It is especially abundant among the roots of 

 oar- weed, and a few specimens should be extricated with 

 care and put into clean water. Notice first the much 

 greater activity than that displayed by starfish ; it is often 

 difficult to say whether a specimen of the latter is alive or 

 dead, so limp and flaccid does it appear even when taken 

 fresh from its pool. The brittle-star, on the other hand, is 

 continually wriggling its arms, and can progress rapidly by 

 their means at a rate which has been estimated at about 



FIG. 42. Common brittle-star (OphiotJirix fragilis). 

 r, radial shield ; sp, spines. 



