276 JELLY-FISH, STAR-FISH, AND SEA-URCHmS. 



eventually to overbalance the system round the 

 fulcrum supplied by the tips of the other two rays, 

 and thus bring the animal down upon its ventral 

 surface. 



But it is in the case of Echinus that these right- 

 ing movements become most interesting, from the 

 fact that they are so much more difficult to accom- 



Fig 48. — Righting movements of Astr.'pecten. 



plish than they are in the case of the Star-fishes. 

 For while a Star-tish is provided with flat, flexible, 

 and muscular rays, comprising a small and light 

 mass in relation to the motive power, an Echinus 

 is a rigid, non-muscular, and globular mass, whose 

 only motive power available for conducting the 

 mancEuvre is that which is supplied by its re- 



