THE ANIMAL KINGDOM. 



the well-known Star-fish (Fig. 4), which belongs to the 

 class Echinodermata,*or spiny-skinned. But this radiat- 

 ing character is not so visible in the outward form of the 

 Echinus or Sea-urchin, which is represented in Fig. 105. 

 When the shell is sawn across, however, the whole 



Fig. 105 SHELL OF THE ECHINUS, OR SEA-URCHIN. 



internal structure of the animal is seen to diverge from 

 the centre. You will remember that an orange presents 

 externally an almost spherical form, but when you cut it 

 across you find that all the parts radiate from one point. 



The Echinus moves in a most wonderful manner. You 

 will observe that its shell is studded with round, knob-like 

 projections ; on these a number of sharp and strong spines, 

 something like those of a hedgehog, are placed, these having 

 a cup-like hollow at their base, so that they work as it were 

 by a ball-and-socket joint ; and by these means the Echinus 

 can not only move along, but can bury itself in the sand. 



hdfiehog ; litnna, skin. 



