108 MEMOIRS OF 



they all have red blood, a muscular heart, a 

 mouth with two jaws, one above, or before the 

 other, distinct organs for sight, hearing, smell, 

 and taste, placed in the cavities of the face, 

 never more than four limbs, the sexes always 

 separated, and a similar distribution of medul- 

 lary masses, and of the principal branches of the 

 nervous system. When thoroughly examining 

 each of the parts of this great series of animals, 

 we shall always find some analogy between them 

 all, even in the species the farthest from each 

 other ; and w^e can follow the gradations of the 

 one same plan, from man to the last of the 

 fishes. In the second form there is no skeleton, 

 the muscles are only attached to the skin, which 

 forms a soft envelope, contractile in various 

 senses, in many species of which are engendered 

 stony plates, called shells, the position and pro- 

 duction of which are analogous to those of the 

 mucous body to which they belong. Their ner- 

 vous system and viscera are contained in this 

 general envelope ; the former is composed of 

 several scattered masses, united by nervous 

 threads, the principal of wlucli, ])laced on the 

 oesophagus, bear the name of brain. In general, 

 they only possess the senses of taste and sight, 

 and even the last is often wanting. Only one 



