

41 



VERTEBRATA. 2 



. 



The circulation of the blood is complete in Vertebrates, 

 the arteries being joined to the veins by capillaries, so 

 that the blood never escapes into the visceral cavity, as 

 in the Invertebrates. All have a portal vein, carrying 

 blood through the liver, and all have lacteals and lym- 

 phatics. The blood is red, and contains both red and 

 white corpuscles. The teeth are developed from the 

 dermis, never from the cuticle, as in Mollusks and Artic- 

 ulates ; the jaws move vertically, and are never modified 

 limbs. The liver and kidneys are always present. The 

 respiratory organs are either gills or lungs, or both. 

 Vertebrates are the only animals which breathe through 

 the mouth. 



The arrangement of the circulatory system varies i:i 

 the different classes, in accordance with 

 the structure of the respiratory organs. 

 In Fishes (Fig. 140) the heart is double 

 as in the Oyster, but instead of drivh\ 

 arterial blood over the body, it receive.; 

 the returning, or venous blood, and sends 

 it to the gills. From the capillaries of 

 the gills the blood is passed into a large 

 artery, or aorta, along the back, which 

 distributes it by a complex net-work of 

 capillaries among the tissues. These cap- 

 illaries unite with the ends of the veins 

 which pass the blood into the auricle of 



the heart, after purification in the liver "; Auri <; le - ^-Ventri- 

 cle, c. Pulmonary Ar- 



and kidneys. tef y- e - Pulmonary 



Veins, bringing blood 



In Amphibia and Reptiles (Fig. 141) from the giiis, </, and 



uniting in the Aorta, 



the heart has three cavities; two auricles /. s . 

 1 



