152 INTRODUCTION TO ZOOLOGY. 



spines. The name means hedgehog-skinned. In the 

 sea urchin, the spines are longer and the appropriate- 

 ness of the name is more clearly manifested. 



We meet an entirely new fact in the study of the 

 starfish. There is neither head nor tail, nor is there a 

 right and a left side. We could divide it into five 

 parts, each part containing one arm, and the parts 

 would be approximately equivalent to each other ; 

 but the parts, or arthromeres, are not arranged seri- 

 ally, one behind another, as are the segments in a 

 grasshopper or an earthworm, but they radiate from 

 a common center. From this fact, animals of this 

 branch were formerly called Radiates. 



An animal that has a right and a left side which are 

 approximately equal to each other, is said to be bilat- 

 erally symmetrical. Bilateral symmetry is the corre- 

 spondence between the right and left sides. All the 

 animals that we have previously studied have been 

 bilaterally symmetrical, except the oyster and the 

 snail. In these instances the symmetry is obscured. 



The starfish is not bilaterally symmetrical, although 

 it is possible to draw a line through the madreporic 

 body that shall divide it into two corresponding 

 parts. The bilateral symmetry is subordinate to the 

 radial arrangement. 



The stomach of the starfish is quite voluminous, 

 and is capable of being thrust out of the mouth and 

 wrapped around an object which it wishes to digest. 

 It is very thin, and can be insinuated into a very 

 small crack between the valves of a clam or mussel 

 or oyster, and these are the animals which constitute 

 the principal prey of a starfish. The intestine is very 

 short, and while it is possible to demonstrate that it 

 has an external opening on the aboral surface, such 

 opening is not readily apparent. 



We are introduced in the starfish to an entirely 

 new method of locomotion. It is a hydraulic, or 



