356 ECHINODERMATA [CH. 



flower, hence the name applied to them, viz. petaloid ambulacra. 

 The special characters of these two orders are as follows : 



II. CLYPEASTKOIDEA or Cake-urchins. They live at or near the 

 surface of the sand. They still retain their teeth, which are placed 

 almost horizontally, and they use them as spades to shovel the sand 

 into the mouth. All the spines covering the upper surface are 

 ciliated and so a constant current of water sweeps over the expanded 

 tube-feet which act as gills. In addition to these tube-feet the 

 whole aboral surface, radii and interradii included, is covered with 

 a multitude of minute tube-feet provided with suckers. Similar 

 tube-feet are found on the oral surface, but they are confined to the 

 radii. This immense multiplication of tube-feet is of course due to 

 the small purchase that any one of them is able to get on such a 

 yielding material as sand. In a word, the animal moves itself by 

 a multitude of minute pulls instead of by a lesser number of stronger 

 pulls as do the Endocyclica. There are calcareous pillars stretching 

 from the upper to the lower surface of the shell or test, apparently 

 to enable them to withstand rough usage, since in many cases they 

 live within reach of the breakers. The best known British species 

 is Echinocyamus pusillus, a little oval Sea-urchin about the size of a 

 pea, whence the common name applied to it, viz. Pea-urchin. On 

 the east coast of North America one species, Echinarachnius parma, 

 the Sand-dollar, is very common ; this is an extremely flattened 

 urchin of circular outline, the shape and size of which have suggested 

 a comparison with the famous silver-dollar of the United States 

 currency. 



III. SPATANGOIDEA or Heart-urchins. These live buried at 

 depths of a few inches to a foot beneath the surface of the mud, 

 and the body is more or less oval or egg-shaped, slightly flattened 

 underneath. The mouth is sometimes in the centre of the under 

 surface and sometimes nearer one end, and is usually crescentic 

 and always without any trace of jaws. These urchins have usually 

 only four of the ambulacra "petaloid"; the fifth has a few long 

 tube-feet with expanded fringed discs. In the case of the familiar 

 British species Echinocardium cordatum it is known that the urchin 

 extends these tube-feet from its burrow up to the surface of the 

 sand and collects with them small organisms lying on the surface. 

 These are pushed within reach of the buccal tube-feet and so 

 reach the mouth. 



The Spatangoidea do not use their tube-feet to walk with, but 

 move by means of spines which are provided with flattened tips, 



