58 INTRODUCTION. 



moistening the eggs. The eye is formed by numerous mem- 

 branes, and covered by the skin, which at this place is trans- 

 parent and which is sometimes so doubled as to form folds or 

 eye-lids. Each eye has a large ganglion giving rise to innu- 

 merable nerves. The ear is a small simple cavity, sunk on 

 each side near the brain without any external canal, where a 

 small membranous bag is suspended that contains a little stone. 

 The brain is contained in a cartilaginous cavity which is a ru- 

 diment of a skull. 



52. Such is the immense series of invertebrate animals. 

 They constitute, as we have seen, three different types. We 

 have also seen that in each type there is a general resemblance, 

 and also different degrees of complication and perfection in 

 their organization. 



The radiata are evidently the most simple. Through some 

 of their number they approximate to the infusorii. Even the 

 most complicated among them have yet no central organ of 

 circulation, and no predominant nervous one: destitute of cen- 

 tral organs, they have no organic or vital unity. 



After the radiata come the mollusca and the articulata. As 

 to the order of organic superiority of these two divisions, it is 

 difficult to determine, for on the one hand, if the articulata are 

 inferior to the mollusca as regards the vegetative organs and 

 functions, since many of them are deprived of a true circula- 

 tion, a function existing in all the mollusca, on the other, the 

 latter are inferior to the former in the development and ap- 

 proximation of the nervous masses, and above all in that in- 

 stinct which is so perfect in some of the articulata, as to bring 

 them near to the vertebrata. 



OF VERTEBRATE ANIMALS. 



53. Vertebrate animals, or as they are termed, the verte- 

 brata, form a type or mode of organization, to which man and 

 those animals that most resemble him belong. They resemble 

 the invertebrate in the organs of the vegetative functions, 

 while they widely differ from them in those of the animal 

 functions. Their external conformation, with the exception 



