312 VERTEBRATA. 



electric telegraph, of which the brain is the central 

 office, and the nerves the wires, along which travel 

 with inconceivable rapidity the various commands 

 and reports from all parts of the complex system. 



With the increased development of the nervous 

 system in the vertebrate classes, the organs of the 

 senses assume a proportionate perfection of structure. 

 The eyes, now invariably two in number, are lodged 

 in cavities formed for their reception in the bony 

 framework of the face. The auditory apparatus, of 

 which only rudiments exist in the lower animals, 

 gradually becomes more and more completely 

 developed. Organs of smell of variable construction 

 are generally present. The tongue becomes slowly 

 adapted to appreciate and discriminate savours, and 

 the sense of touch is especially conferred upon organs of 

 different kinds peculiarly fitted to exercise the faculty. 

 Thus, with increased intelligence, higher capabilities 

 of enjoyment are allotted, and sagacity develops itself 

 in proportion as the nervous centres expand. 



The blood of all the vertebrata is red, and con- 

 tains microscopic corpuscles of variable form and 

 dimensions in different animals. In the class of 

 fishes, owing to the as yet imperfect condition of the 

 respiratory apparatus, the temperature of the body is 

 scarcely higher than that of the surrounding medium ; 

 and even in reptiles such is the languid condition of 

 the circulation, and the incomplete manner in which 

 the blood is exposed to the renovating influence of 

 oxygen, that the standard of animal heat is still 

 extremely low ; but in the higher classes, the birds 

 and mammals, the effect of respiration is increased 

 to the utmost, and pure arterial blood being thus 

 abundantly distributed to all parts, heat is more 

 rapidly generated, the warmth of the body becomes 

 considerably increased, and such animals are perma- 

 nently maintained at a higher temperature than that 

 of the medium in which they live. Hence, the dis- 

 tinction generally made between the hot-blooded and 

 the cold-blooded vertebrata. 



