10 FIELD ZOOLOGY. 



corpuscles, not to speak of functions attributed to other 

 organs of ours, as those organs that give us the sensations 

 of light and darkness, heat and cold, all of them differ- 

 entiated parts of our complex nervous system. The cells 

 of the interior layer of the body, the entoderm, perform 

 the functions of our highly elaborated alimentary system, 

 while the skeleton of the sponge is the product of the 

 working energy of the middle layer of cells, the mesoderm. 

 This general division of functions holds true for other 

 animals from the sponge to the complex animals; the 

 primitive ground sense, not localized, but distributed 

 over the body surface in the simple animal, becomes the 

 nervous system of the more complex animal and is there 

 differentiated into touch, sight, taste, hearing, smell, 

 temperature, and other sensations believed to be inter- 

 preted by the reflex or the spinal system. But an organ, 

 on its first appearance, is little differentiated from the 

 surrounding tissue, and only gradually reaches higher 

 levels of perfectness; hence the simple eye is probably an 

 earlier expression of a light sensitive, chiefly useful in 

 distinguishing light from darkness; while the compound 

 eye may be said to represent a later and more complex 

 organ of sight. The function of the simple eye is not 

 certainly known. In those animals which see most 

 definitively, that is, which come nearest to forming a 

 perfect image, the simple eye has gone through no develop- 

 ment from its primitive structure, while the compound 

 eye, in these insects, has the greatest number of facets, 

 seeming to indicate that the compound eye is the effective 

 organ of sight. 



Again, as to the simple eye : wherever in any animal, 

 simple or complex, pigment is present in the skin cells, 

 those cells must be sensitive to light; that they are so 



