74 AFFINITY WITH STAR-FISHES. 



which they are applied, of enabling the animal to sink 

 itself in the sand, shovelling the fine particles out of 

 the way, and throwing them over its back. When 

 thrown upon shore, the spines are usually more or less 

 broken, and soon are completely worn off, when the 

 dead shell resembles a heart-shaped egg, of a dirty- 

 white colour, frosted over with minute tubercles, which 

 are largest on its under surface, where the orifice of the 

 mouth is seen ; and it is marked, both on the back and 

 lower surface, with five radiating smooth depressions, 

 bordered with a double row of pin-holes. These spaces, 

 which are much more developed on the back than on the 

 oral surface, are called ambulacra ; and through the 

 pores or pin-holes which border them, the animal pro- 

 trudes long worm-like suckers, which serve the office of 

 feet, and enable him to move about by a sort of warp- 

 ing motion (to speak nautically), fixing the sucker of 

 one fibrous cord in advance of his position, gradually 

 bringing the rest forward, and so dragging the body 

 along. Those on the oral surface are much less deve- 

 loped, and chiefly serve to hold the ground. It is 

 curious to find a creature whose organs of locomotion 

 are most developed on the upper surface ; but we may 

 be assured that they are not so placed without a wise 

 design. It is easy to see that such an organization en- 

 ables the creature to recover its natural position with 

 ease, if accidentally inverted ; but the arrangement pro- 

 bably serves many other purposes. 



The affinity of the Heart Urchin with the common 

 Egg Urchins is readily seen ; their connection with 

 Star-fish is, at first sight, less obvious. Nevertheless, a 



