SEA URCHINS. 65 



scarlet lines running along the rays and continued round 

 the edges of the connecting membrane. 



Next, let us imagine that the calcareous skeleton, 

 instead of being composed of movable parts, be soldered 

 together, so that the rays can no longer be bent, we 

 shall have the Cuke Urchin, or Shield Urchin (Ecliinar- 

 achnius), so called on account of its flatness. 



For the next step, let us suppose that before the 

 change of skeleton takes place, the Bird's Foot Starfish 

 were rolled up, so as to bring the tips of the rays in con- 

 tact, we should have a creature which is well known as 

 the common Sea Urchin or Sea Egg (Echinus). 



In these creatures ambulacra still retain their places, 

 but are not nearly so conspicuous as in the Starfishes, 

 especially as they are nearly hidden by the remarkable 

 appendages which have earned for the creatures the title 

 of Sea Urchins or Hedgehogs. 



The whole external surface of the Sea Urchin is 

 covered with radiating spines very much resembling 

 those of the hedgehog, but having a much more com- 

 plex mechanism, and being capable of movement in any 

 direction. How they are moved, we shall now see. 



We are all familiar with the " ball-and-socket " joint, 

 which is so much used in machinery. Carrying out what 

 I believe to be an universal rule, that all mechanical 

 inventions of men have their prototypes in Nature, the 

 spines of the Echinus afford an absolutely perfect example 

 of the ball-and-socket joint. 



Upon nearly the whole external surface of the Sea 



