INTRODUCTION. 



branchiate : * thus, at a certain epoch they undergo a metamorphosis ; the 

 gills become obliterated, the lungs developed, the heart acquires three 

 cavities, viz., two auricles, and one ventricle. The perennibranchiate -f 

 Amphibia, as the Proteus, the Axolotl, and the Siren, though they 

 acquire lungs, do not lose their branchiae ; they are permanently 

 aquatic. The skin of the Amphibia is a highly sensitive and secreting 

 surface, and the blood, circulating through its vessels, becomes partially 

 arterialized ; hence it compensates for the limited use of the lungs, 

 during submersion, or hibernation. The toes are destitute of claws ; 

 there is no external auditory apparatus ; the brain in the perennibranchiate 

 group, and in the larva or tadpole state of those which, as the Frog or 

 Newt, lose the gills, resembles that of the fish : as the metamorphosis 

 of the tadpole takes place, and the legs and arms become developed, the 

 cerebral hemispheres enlarge, but present neither convolutions nor ven- 

 tricles or internal cavities. Limbs variable, four or two. 



REPTILIA. Vertebrate animals, with cold and red blood, with true 

 lungs ; oviparous : the heart has two auricles and one ventricle ; the latter 

 cavity receives both the venous and the arterialized blood, and is divided 

 more or less by an ascending imperfect valve ; the body is covered with 

 scales, or horny plates ; there is either no external auditory apparatus, or 

 only a simple orifice, open in some, in others covered with a transparent 

 tympanic membrane, and, in a few, as the Alligators, protected by a move- 

 able valve. Brain, with the cerebral hemispheres exceeding the optic 

 lobes in magnitude, and containing each a distinct ventricle ; the cere- 

 bellum remarkable for its proportionate smallness. Limbs variable, 

 four, or two, or wanting. 



AVES (Birds). Vertebrate animals, with warm and red blood ; ovi- 

 parous : heart with four cavities, viz., two auricles and two ventricles, 

 whence there is a distinct pulmonic and systemic circulation ; lungs 

 ample, but fixed, and communicating with extensive air-cells, and also with 

 the cavities of the bones of the extremities, which (except in a few, as the 

 Penguins and great Auk) are destitute of marrow. The brain fills the 

 cavity of the cranium ; the hemispheres extend backward, cover the 

 optic lobes, and come into contact with the enlarged and sulcated, or 

 furrowed, cerebellum, but they are still destitute of convolutions ; the 

 optic lobes are large, but separated from each other to the sides of the 

 medulla oblongata ; the olfactory tubercles are small. The eyes have 

 not only eyelids, but a third membraneous covering, or membrana nic- 

 titans, capable of being drawn over the eye at pleasure ; the ears are 

 open externally ; the jaws are covered with a horny sheath, forming a 

 beak. There is no epiglottis. The limbs are always four, the anterior 



* Having perishable gills, from caducus, perishable, bronchia, a gill. 

 t Having permanent gills (perennis, permanent). 



