20 THE ANIMAL KINGJJOM. 



composition to bone, but which grow from the jaws by a process 

 of secretion. The jaws of one entire class, however, (that of 

 Birds) and the genus Testudo in that of Reptiles, are invested 

 with a horny substance. 



The intestinal canal extends from the mouth to the anus in 

 various degrees of expansion or contraction. It possesses cer- 

 tain appendices, and receives liquids of a solvent nature, viz. 

 saliva from the mouth, the secretion of the gland denominated 

 pancreas, and the bile, which is produced by another large gland, 

 the liver. 



In the passage of the food through the alimentary canal, the 

 part of it adapted to the purposes of nutrition, and termed the 

 chyle, is absorbed by the lacteal vessels, and conveyed into the 

 pulmonary artery, where, in combination with the blood, it un- 

 dergoes a certain change ; and after each portion of the body has 

 received its proper supply, the remainder is carried back into the 

 veins by a set of vessels analogous to the lacteal, and which to- 

 gether form what is usually called the lymphatic system. The 

 veins carry back to the heart the blood which has served the pur- 

 poses of nutrition. This blood, however, must pass either wholly 

 or partially into the organ of respiration for the purpose of re- 

 suming its arterial character, before it is carried back by the arte- 

 ries to the different parts of the body. In the three first classes 

 of vertebrated animals the organ of respiration consists of lungs, 

 an assemblage of small cells permeable by the external air. In 

 fishes alone respiration is performed by gills or branchicea 

 series of laminae between which the water passes. 



In all vertebrated animals, the blood which furnishes to the 

 liver the materials of the bile is supplied from the venous blood 

 which has circulated in the intestines, and which, after being 

 reunited in a trunk called the vena porta, is again divided at 

 the liver, and distributed in ramifications through its substance. 



The sexes in this division are always in separate individuals ; 

 but the mode in which fecundation is performed is different in 

 the various classes. 



Though in all these points the vertebrated animals have a 

 general resemblance, yet the various beings of which this divi- 

 sion is composed present peculiarities, which are the foundation 

 of their arrangement into classes. These differences depend 

 upon the nature and energy of their movements, which again 



