1302 THE STORY OF THE UNIVERSE 



tribution 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 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 mus- 

 cles are merely attached to the skin, which constitutes 

 a soft contractile envelope, in which, in many spe- 

 cies, 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 aesophagus 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 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 di- 

 gestion and secretion are nearly as complex as in the 

 vertebrata. We will distinguish the animals of this 

 second form by the appellation of Animalia Mol- 

 lusca. 



Although, as respects the external configuration of 

 the parts, the general plan of their organization is 

 not as uniform as that of the vertebrata, there is al- 

 ways an equal degree of resemblance between them 

 in the structure and the functions. 



