180 DENTAL APPARATUS 



faculty of secreting the carbonate of lime for the fonna- 

 tion of shell, and this, as fast as it is secreted, is depo- 

 sited, layer after layer, round the edges of each plate, 

 so that these are constantly enlarging during the active 

 growth of the animal, till it has attained its full size. 

 The spines are deposited in the same way, by the mem- 

 brane which clothes them, and exhibit, in their struc- 

 ture, as seen in the microscope, the most exquisitely 

 beautiful and regular arrangement of particles that can 

 well be conceived. Under the ceaseless activity of the 

 vital power, the deposition of shelly matter proceeds, 

 and results in a structure whose exactness and beauty 

 it would be vain to imitate. 



The bony contents of the shell are not at all less ela- 

 borate ; and when we consider the apparently low grade 

 of the animal for whose use they are designed, and the 

 simplicity of other parts of its organization, we cannot 

 fail to be struck with wonder at the amount of skill and 

 contrivance lavished on its dental apparatus. The pre- 

 hension of food is certainly the first requirement of 

 animal life, and consequently we find the organs con- 

 nected with its mastication and digestion, the mouth 

 and stomach, those that are most prominently deve- 

 loped in the lower animals. Some of these appear to 

 be mere stomachs, endowed with a capacious gullet, and 

 a set of sucking lips. In others, the organs for cutting 

 or tearing food, or for grinding it to jelly, are extremely 

 powerful ; and when their strength is compared with 

 the weakness or inertia of their other organs, they give 

 us that impression of disproportion which leads to the 

 idea of monstrosity, and which may, in some degree, 



