16 ZOOLOGY. 



is due to the globules called corpuscles (Lat. corpus, a 

 body ; cle, little), and not to the fluid in which they are 

 suspended, this being colourless. It circulates to all parts 

 of the body, in a system of close vessels, called veins and 

 arteries. The arteries convey the blood from the heart, 

 the veins carry it back. 



There are always special respiratory or breathing 

 organs for the purification of the blood. In mammals, 

 birds, and reptiles, the oxygen required for this purpose 

 is obtained directly from the atmosphere. Their breath- 

 ing organs are called lungs. Fishes breathe by gills, and 

 obtain the oxygen from the air dissolved in water. The 

 amphibia breathe by gills when immature, and by lungs 

 in the adult state. In all cases, the organs of respiration 

 are connected with the commencement of the digestive 

 tube. 



In all the vertebrates the male and female organs are 

 found in distinct individuals. 



DIVISIONS OP THE VEKTEBRATA. 



10. This sub-kingdom is divided into five classes : 



1. Mammalia (warm-blooded quadrupeds which suckle 

 their young). 



2. Aves (Birds). 



3. Reptilia (Reptiles). 



4. Amphibia (Frogs, Toads, &c.) ; and 



5. Pisces (Fishes). 



11. The Amphibia and Pisces agree in possessing gills 

 during either the whole or a part of life j the embryo is 

 destitute of the membranous sac, termed the amnion 

 (Gr. amnos, a lamb), and of the structure called the 

 allantois (Gr. alias, a sausage) ; the red blood-corpuscles 

 are oval in shape, and contain a nucleus. These two 

 classes, according to Professor Huxley, form the 

 division Ichthyopsida, (Gr. ichthus, a fish ; opsis, appear- 

 ance). 



12. The Eeptilia and Aves have also nucleated red blood- 



