276 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 
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 Astropecten. 
plish than they are in the case of the Star-fishes. 
For while a Star-fish 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- 
