162! PHILOSOPHY OF ZOOLOGY. 



extremities, where present, never exceed four in number, and 

 are more or less intimately articulated to the vertebral co- 

 lumn. 



3. Muscular System. The muscles in this group of ani- 

 mals, are principally supported by the osseous system. 

 Those which are destined for the motion of the extremities, 

 have their origin and insertion in the bones by which these 

 are supported, or in the portion of the skeleton to which 

 they are articulated. In this peculiar character, they differ 

 widely from the invertebral animals. 



4. Nervous System. The peculiar characters of the ner- 

 vous system of vertebral animals, have been already given, 

 in considerable detail, in the preceding volume, p. 150. 



5. Digestive System. The opening of the mouth is al- 

 ways transverse, the motion of the jaws being performed in 

 the same plane. The liver is always present. 



6. Circulating System. Lacteals, lymphatics, and veins, 

 occur as absorbents, and are aided by a pulmonic auricle 

 and ventricle. Where a systemic heart is present, it is al- 

 ways in contact with the pulmonic. The blood is uniform- 

 ly of a red colour. The kidneys always occur for the se- 

 cretion of the urine. 



7. Reproductive System. The sexual organs are usually 

 placed on different individuals, and the females are vivipa- 

 rous, oviparous, or ovoviparous. 



The subdivision of the vertebral animals is easily ac- 

 complished, from the characters furnished by the Circula- 

 ting System. In one group, there is a perfect systemic and 

 and pulmonic heart, with warm blood ; while, in the other, 

 the heart is more or less defective, and the blood is cold. 



Vertebral Animals with Warm Blood. 



As the temperature of the body is usually higher than 

 the surrounding medium, the skin is either furnished ex- 



