SUCKERS AND LOCOMOTIVE SPINES. 221 



suppose a specimen at rest. All the suckers are re- 

 tracted within, or close to, the shell, and the spine 

 remains motionless. A few of the suckers now pro- 

 trude; they search around for a solid foundation; 

 others follow them, until at length many of these 

 adhesive discs are at work, and the animal is fixed 

 securely by their assistance. Should the Echinus 

 wish to change its place, the suckers behind are seen 

 to relax their hold, while the contraction of others in 

 front brings forward the shell. The suckers therefore 

 in this case are the principal instruments of pro- 

 gression, wherein they are but feebly aided by the 

 spines, and thus the creature advances in its course 

 with surprising expedition. If a portion of food be 

 now dropped into the vessel, the Echinus speedily be- 

 comes sensible of its presence. Should the creature 

 be above, the suckers are soon seen to be put into 

 active requisition, and its descent commences imme- 

 diately as it hastens to the repast. 



The most striking feature in the construction of 

 the Sea-urchin, from which indeed the animal de- 

 rives its name, is found in the prickles or spines which 

 stud the exterior of its shelly box, and which form a 

 very important part of its locomotive machinery. 

 Each of these spines is in fact a kind of leg, articu- 

 lated to the exterior of the shell by a beautifully con- 

 structed ball-and-socket joint, and moved by a special 

 apparatus ; so that by means of these numerous legs 

 the creature is enabled to roll itself about upon the 

 beach, or, if danger threaten, to bury itself in the 

 sand, and thus escape from observation. This latter 

 feat the Echini are in the habit of putting in practice 



