34 INTRODUCTION. 



on the development or addition of certain parts, which produce no 

 essential change in the plan itself. 



In the first of these forms, which is that of Man, and of the ani- 

 mals most nearly resembling him, the brain and principal trunk of 

 the nervous system are enclosed in a bony envelope, formed by the 

 cranium and vertebrae; to the sides of this intermedial column are 

 attached the ribs, and bones of the limbs, which form the frame 

 work of the body; the muscles generally cover the bones, whose 

 motions they occasion, while the viscera are contained within the 

 head and trunk. Animals of this form we shall denominate 



Animalia Vertebrata, 



Or vertebrated animals. They have, all, red blood, a muscular 

 heart, a mouth furnished with two jaws, one situated either above or 

 before the other, distinct organs of sight, hearing, smell and taste 

 placed in the cavities of the face; never more than four limbs, the 

 sexes always separated, and a very similar distribution of the me- 

 dullary masses and the principal branches of the nervous system. 



By a closer examination of each of the parts of this great series 

 of animals, we always discover some analogy, even in species the 

 most remote from each other; and may trace the gradations of one 

 same plan from Man to the last of the Fishes. 



In the second form there is no skeleton; the muscles are merely 

 attached to the skin, which constitutes a soft contractile envelope, in 

 which, in many species, are formed stony plates, called shells, whose 

 position and production are analogous to those of the mucous body. 

 The nervous system is contained within this general envelope along 

 with the viscera, and is composed of several scattered masses con- 

 nected by nervous filaments; the chief of these masses is placed on 

 the resophagus, and is called the brain. Of the four senses, the 

 organs of two only are observable, those of taste and sight, the lat- 

 ter of which are even frequently wanting. One single family alone 

 presents organs of hearing. There is always, however, a complete 

 system of circulation, and particular organs for respiration. Those 

 of digestion and secretion are nearly as complex as in the Vertebrata. 

 We will distinguish the animals of this second form by the appella- 

 tion of 



