VERTEBRATED ANIMALS. 325 



delicate texture, and which require to be shielded 

 from the slightest pressure. This occurs with re- 

 gard to the brain, and the spinal marrow, which 

 we shall presently find are specially guarded by a 

 bony structure, enclosing them on every side, and 

 forming a solid case for their protection. The 

 mass of bone, thus provided to defend the brain, 

 gives also the opportunity of lodging safely the 

 delicate apparatus subservient to the finer senses, 

 namely, those of sight, of hearing, and of smell. 

 The security which these organs derive from this 

 protection allows of their being carried to a higher 

 degree of improvement than could be attained in 

 the lower orders. 



There is also another advantage, of considerable 

 moment, which residts from the internal situation 

 of the skeleton, namely, that it admits of an in- 

 definite extension by growth, without interfering 

 with the corresponding enlargement of the softer 

 organs ; for we have seen that in all the instances 

 in which this arrangement is reversed, that is, 

 whenever the enclosing surfaces become solid, and 

 can no longer yield to the dilatation of the contained 

 organs, no alternative remains but that of breaking 

 up the exterior case, and wholly casting it off, to 

 make room for the further growth of the animal ; 

 after which operation, it has to be replaced by 

 another covering of larger dimensions. This ope- 

 ration is generally required to be performed a great 

 number of times, before the animal can acquire the 

 size it is destined to attain. Hence the frequent 

 moultings of the caterpillar ; hence the repeated 

 castings of the shells of the Crustacea ; and hence 

 also the successive metamorphoses of the insect. 



