VERTEBRATED ANIMALS. 363 



ing no intermediate islands from which we 

 might gather indications of these tracts of land 

 having been originally connected. At the very 

 first sight indeed, the general fabrics of these 

 two descriptions of animals appear to have been 

 constructed upon opposite principles ; for in the 

 one, as we have already seen, the softer parts are 

 internal, and are enclosed in a solid crust, or 

 shell, or horny covering, answering at once the 

 purposes of protection and mechanical support, 

 and furnishing extensive surfaces for the attach- 

 ment of the organs of motion. But in the Verte- 

 brata, the solid frame work which serves these 

 purposes occupies, for the most part, an internal 

 situation, constituting a true jointed skeleton, 

 which is surrounded by the softer organs, and to 

 which the muscles, destined to move their several 

 parts, are attached. The office of external de- 

 fence is entrusted solely to the integuments, and 

 their different appendages. Such is the general 

 character of the arrangements which nature has 

 here adopted ; from which, however, she has oc- 

 casionally deviated with respect to some import- 

 ant organs of extremely delicate texture, and 

 which require to be shielded from the slightest 

 pressure. This occurs with regard to the brain, 

 and the spinal marrow, which we shall pre- 

 sently find are specially guarded by a bony 

 structure, enclosing them on every side, and 

 forming an impenetrable case for their pro- 



