124 THE STORY OF ANIMAL LIFE 



also purplish-red and' red. The spines are 

 mounted on something resembling a ball and 

 socket joint, with a ring-shaped pad, so that they 

 have a wide range of movement; if any of the 

 spines are touched they are immediately set back 

 over a considerable part of the neighbouring 

 surface. 



Other kinds may be found upon a more sandy 

 shore. These are heart-shaped and much lighter 

 in colour. The shell is thinner and of less weight. 

 These adaptations for lessening the animal's 

 weight enable it to move over sand : the spe- 

 cies above described has no occasion for such 

 precautions. When it crawls over rocks and the 

 strong seaweeds that grow on them, there is no 

 fear of its sinking in. The sand-dweller, on the 

 contrary, must take care that it is not swal- 

 lowed up. 



There are Sea-Urchins that carry their pre- 

 cautions against sinking to an extreme degree. 

 These are the Shield-Urchins or Clypeastridae, 

 so-called from their flat shape ; they include the 

 American forms popularly known as " sand- 

 cakes." The diagram (Fig. 36) shows one of the 

 most curious of these flattened forms adapted for 

 moving over fine sand and ooze, and literally " as 

 flat as a pancake." The mouth is approximately 

 in the centre of the lower surface, B ; the upper 

 surface, A, shows a rosette pattern on the top of 

 the shell. This is formed by the rows of holes 

 for the very minute tube-feet. In the English 

 Sea-Urchin above described, which is one of the 

 group called (for that reason) Regulares, the rows 

 of holes are uniformly continued all along the 

 rounded sides of the body down to the neighbour- 

 hood of the mouth. Here they are much re- 



