SEA TJECHINS. 69 



ambulacra, just as the Starfish does. These wonderful 

 instruments of locomotion are kept within the body until 

 they are wanted, and then they are protruded as they 

 are wanted. As they must necessarily pass beyond the 

 spines, they are of considerable length when extended to 

 the utmost. 



Generally the Echinus prefers to use those ambulacra 

 Avhich are situated round the mouth. If specimens be 

 kept in an aquarium, they will move freely about, and it 

 is a very interesting sight to watch them gliding along 



Fig. 4. SHELL OF SEA URCHIN. 



the glass with a movement almost as slow and regular as 

 that of the minute-hand of a watch. 



The apertures through which they pass are best seen 

 by opening the shell, holding it up to the light, and 

 looking at it from the interior. They can be very well 

 distinguished, looking like rows of very fine needle-holes. 

 Even in the largest specimens these holes are exceedingly 

 minute. The mechanism of the ambulacra themselves is 

 precisely the same as has been described in another 

 chapter. 



