VI EXPLANATION OF THE FIGURES. 



age ; while only a few have continued from the Silurian, through every 

 period to the present. Thus, the Crinoids were very numerous in the 

 Primary Age, and are but slightly developed in the Tertiary and Modern 

 Age. It is seen, at a glance, that the Animal Kingdom is much more diver- 

 sified in the later, than in the earlier Ages. 



Below the circle is a section, intended to show more distinctly the re- 

 lative position of the ten principal formations of stratified rocks (461), com- 

 posing the four great geological ages ; the numerals corresponding to those on 

 the ray leading to Man, in the circular figure. See also figure 154. 



The CHART OF ZOOLOGICAL REGIONS, page 163, is intended to show the 

 limits of the several Faunas of the American Continent, corresponding to the 

 climatal regions. And as the higher regions of the mountains correspond in 

 temperature to the climate of higher latitudes, it will be seen that the northern 

 temperate fauna extends, along the mountains of Mexico and Central Amer- 

 ica, much farther towards the Equator, than it does on the lower levels. In 

 the same manner, the southern warm fauna extends northward, along the 

 Andes. 



FIG. 



1. Simple cell, magnified, as seen in the house-leek. 



2. Cells when altered by pressure upon each other ; from the pith of elder. 



3. Nucleated cells (#), magnified ; b, nucleolated cells. 



4. Cartilaginous tissue from a horse, magnified 120 diameters. 



5. Osseous tissue from a horse, magnified 450 diameters. 



6. Nervous fibres, showing the loops as they terminate in the skin of a frog. 



7. Gray substance of the brain, magnified. 



8. Head of an embryo fish, to show its cellular structure throughout. 



9. Diagram, to show the nervous system of the Vertebrates, as found in 



a monkey. 



10. Diagram of the nervous system of the Articulates, as seen in a lobster. 



11. Diagram of the nervous system of the Mollusks, as found in Natica lieros. 



12. Diagram of the nervous system of the Radiates, as found in Scutella 



(Echinarachnius par ma) . 



13. Section of the eye. a, optic nerve ; b, sclerotic coat ; c, choroid coat ; 



d, retina ; e, crystalline lens ; /, cornea ; g, iris ; k, vitreous body ; 

 ij chamber, divided by the iris. 



14. Diagram, showing the effect of the eye on rays of light. 



15. Position of the eye of the snail. 



16. Eyes (ocelli) of a spider. 



17. Eye-spots of a star-fish (Echinaster sanguinolentus) . 



18. Compound eyes, showing the arrangement of the facettes, and their con- 



nection with the optic nerve, as seen in a crab's eye. 



19. Diagram of the human ear, to show the different chambers, canals, and 



bones. 



