X INTRODUCTION. 



served further, that in animals of the first kind the blood is 

 always red ; in those of the second kind it is with a few ex- 

 ceptions white. 



In those of the first kind, there is always a bony case, 

 called the cranium, or skull, which contains the brain ; and a 

 number of bones called vertebrae, connected together so as to 

 form a long column, usually called the spine, the backbone, or 

 the vertebral column. This column contains a canal extend- 

 ing its whole length, which receives the spinal nerve or mar- 

 row, as it passes out of the skull, and conveys it along the 

 trunk, to be thence distributed to the various parts of the 

 body. It is, as it were, the main pillar or common sup- 

 port of all the rest of the skeleton ; and hence the animals 

 possessed of it are called VERTEBRAL Animals, as this forms 

 the most striking characteristic which is common to them 

 all. 



In animals of the second kind there is no skeleton and of 

 course no vertebral column. The brain and nervous system 

 are not therefore protected by any bony covering. These 

 organs do not resemble the corresponding ones of the verte- 

 bral animals ; they are less distinct and apparently less im- 

 portant. They have not many common points of resemblance, 

 but as none of them possess a backbone or a skeleton, they 

 are denominated from this circumstance invertebral animals, 

 that is, without vertebrae. 



The first two grand kingdoms of the animal world then 

 are, I. Vertebrated Animals, such as man, quadrupeds, birds, 

 fishes, &c., having a skeleton and red blood ; and, II. Inver- 

 tebrated Animals, such as worms, insects, shell-fish, &c., hav- 

 ing no skeleton and white blood. 



But in examining the first grand kingdom further differ- 

 ences are observed. Man, quadrupeds, whales, and birds 

 have all a temperature above that of the air or water in 

 which they reside. Their flesh is warm, and as this warmth 

 is supposed to depend upon the temperature of the blood, they 

 are called warm-blooded. On the other hand, frogs, toads, liz- 

 ards, serpents, and fishes have all a temperature varying but 

 little from that of the air or water in which they live. They 

 impart when touched the sensation of cold. Hence they are 

 called cold-blooded. Here, then, is afforded ground for a sub- 

 division of the vertebrated animals into the warm-blooded 

 and the cold-blooded. 



