186 



SEA-URCHINS. 



test of the urchins, which varies from a shape somewhat 

 resembling a flattened orange to a heart- shape, or even to 

 a thin disc-like plate. The ordinary urchins shaped 

 somewhat like an orange are, however, those best adapted 

 for gaining a general idea of the structure of the group, 

 and we shall accordingly commence with them. 



If, then, we examine such a test, we shall find that it 

 has an aperture at each of the two poles ; while it is 

 divided into a series of meridional areas, each composed 

 of a number of separate oblong calcareous plates, fitting 

 accurately with one another at their edges, where they are 

 united by a thin membrane. The upper surface of such a 

 test is shown in Fig. 57, from which it will be seen that 



the upper or apical 

 pole, as it may be con- 

 veniently called, con- 

 sists of five somewhat 

 heart - shaped plates 

 surrounding a small 

 circular orifice which 

 is closed with mem- 

 brane in the living 

 state. This central 

 orifice is the vent, 

 while the five plates 

 forming a star sur- 

 rounding it, which 

 generally fall out in 

 the dry state, consti- 

 tute the apical disc. 

 At the opposite or 

 lower pole, we shall 

 find a much larger 

 aperture, which forms the creature's mouth ; and we may 

 observe that just inside this aperture of the test in the com- 

 plete animal there will be found a very complicated cal- 

 careous masticating apparatus known as Aristotle s lantern. 

 Putting aside for the present the apical disc, we may 

 devote somewhat fuller attention to the main body of 

 the test, technically termed the corona. As we have said, 



FIG. 57. Upper Surface of the Test 

 of the Common Sea-Urchin. #, ambu- 

 lacral areas ; i a, intermediate areas. 



