568 THE WONDERS OF GEOLOGY. Lect. V- 



secondary epochs disappears, I will here offer a few 

 general observations on this class of animals, that the 

 unscientific inquirer may be enabled to comprehend the 

 inferences that arise from the facts that have been submitted 

 to his notice.* 



All animals possess organs by which a certain change is 

 effected in the circulating fluid, to refit it for the purposes 

 of nutrition. Mammalia, birds, and reptiles, are furnished 

 with an apparatus of cellular tissue, termed lungs, by which 

 a large surface of the blood is brought in contact with the 

 air ; in aquatic vertebrata this apparatus is the gills, which 

 are organs fringed with innumerable processes, supplied 

 by myriads of vessels, disposed like net- work, by which the 

 blood is exposed to the action of aerated water, oxygen ab- 

 sorbed, and vitality maintained. In Reptiles, the respiratory 

 organs are less developed than in any of the other verte- 

 brated animals, but they all possess lungs, and are capable 

 of breathing air : and some have gills, and perform 

 aquatic respiration. The heart, which is generally three- 

 chambered, is so disposed, that at each contraction only a 

 portion of the volume of blood is sent to the lungs ; hence 

 the action of oxygen on the circulating fluid is in a less 

 degree than in any of the mammalia, birds, or fishes. As 

 animal heat, the susceptibility of the muscles to nervous 

 influence, and even the nature of the skin, are dependent on 

 respiration, the temperature of Reptiles is low, and their 

 muscular powers are, on the whole, very inferior to those of 

 Birds or Mammalia ; requiring no integuments, as hair, wool, 

 or feathers, to preserve their temperature, they are merely 

 covered with scales, or have a naked skin. As they can 

 suspend respiration without arresting the course of the 

 * See my " Medals of Creation," vol. ii. chap. xvii. ; where the subject 

 is so fully considered that a brief notice only is here necessary. For a 

 comprehensive and philosophical view of this department of Palaeon- 

 tology, the English reader should study the memoirs of Prof. Owen, in 

 Brit. Association Reports, 1840, 1 8 i 1 . 



