176 AN INTRODUCTION TO ZOOLOGY. 



areas are modified and interrupted, and have a special 

 rosette round the apex, which marks the situation o£ 

 an area of modified ambulacral tubes; and from the 

 flower-like shape of this rosette these urchins are said 

 to be Petaloid. This is the case in the Heart 

 Urchins, order Spatangidw, of which the common 

 Heart-urchin {Amphidotus cordatus) of the English 

 coast is an example, and in the Flat Urchins, order 

 Clypeastridw. One of the most familiar of the latter 

 is the Sand- cake of the North American coast, which 

 is as flat as a thin biscuit. These various forms are 

 correlated with the habits of the animal ; the very 

 round urchins, like Echinus, can walk about among- 

 stones, like starfishes; the Heart-nrchins live partly 

 buried in sand or mud, and are a transition towards 

 the flat ones, which are suited for lying flat on sand. 

 Many animals that live on sandy parts of the sea 'bed, 

 e.g., the flat-fishes, soles and flounders, and some of the 

 bivalve shell-fish, are flattened, on the same principle 

 that snow-shoes are made large and flat, viz., in order 

 to distribute the weight of the animal over as large an 

 area as possible, and thus prevent it from sinking in. 



The larva of the Echinids is called a Pluteus 

 (easel-shaped) larva, because it has long straight 

 appendages sticking out at angles with one another. 

 These arms, as they are called, are supported by a 

 provisional skeleton, and there are several pairs of 

 them. When the adult Echinus is formed within, 

 the arms atrophy {i.e. are absorbed). Some sea- 

 urchins have the spines arranged so as to form an 



