INTRODUCTION. 25 



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 sepa- 

 rated, and a very similar distribution of the medullary 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 re- 

 mote 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 at- 

 tached 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 connected by 

 nervous filaments ; the chief of these masses is placed on the oesophagus, 

 and is called the brain. Of the four senses, the organs of two only are 

 observable, those of taste and sight, the latter of which is even fre- 

 quently wanting. One single family alone presents organs of hearing. 

 There is always, however, a complete system of circulation, and par- 

 ticular 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 appellation of 



ANIMALIA MOLLU8CA, 



Or soft animals. Although, as respects the external configuration 

 of the parts, the general plan of their organisation is not as uniform as 

 that of the Vertebrata, there is always an equal degree of resemblance 

 between them in the structure and the functions. 



The third form is that remarked in Worms, Insects, &c. Their 

 nervous system consists of two long cords, running longitudinally 

 through the abdomen, dilated at intervals into knots or ganglions. 

 The first of these knots, placed over the oesophagus, and called the brain, 

 is scarcely any larger than those that are along the abdomen, with 

 which they communicate by filaments that encircle the resophagus like 

 a necklace. The covering or envelope of the body is divided by trans- 



