THE GENERAL ANIMAL FUNCTIONS. 



77 



FIG. 41. Diagrams showing some of the stages in the increasing complexity of the 

 simple eye in Invertebrates. A, simple pigment spot in epithelium having nerve-endings 

 associated with pigment cells (as in some medusae) ; B, pigment cells in a pit-like de- 

 pression (as in Patella') ; C, with pin-hole opening and vitreous humor in cavity (as 

 in Trochus) ; D, completely closed pit, with lens and cornea (as in Triton and many 

 other Mollusks) ; E, pigment area elevated instead of depressed, lens of thickened 

 cuticula (as in the Medusa, Lizsia) ; F, retinal cells more highly magnified, ep., 

 epidermis; f, nerve fibre; /, lens; op, optic nerve; p, pigment cells; r, retina; v.h,, 

 vitreous humor. 



Questions on the figures. What changes take place in the sensory 

 epithelium in this series of figures? What is gained by such a depression 

 as occurs in B? What purpose is served by the pinhole and the vitreous 

 humor of C? Describe the change from C to D. What is gained? What 

 may be the function of the pigment? Compare texts. In what respects 

 does E differ from the other types? What two types of cells are figured 

 as belonging to each retina? What constitutes the retina? 



it illustrates the principles made use of in the construction of the photo- 

 graphic camera. In this there is a lens or body which refracts the rays 

 of light in such a way that all the rays passing from a point in the object 

 are brought to a focus at a point on the retina. Another type of image- 

 forming eye is the compound eye of insects and Crustacea (Fig. 42). 

 These are made up of a large number of eye elements each structurally 

 complete in itself whose separately formed images must nevertheless be 

 joined in order to form a picture. 



The degree to which the color-sense is developed among lower animals 

 is very uncertain. The simplest animals may respond differently to light 

 of different colors, but this is a very different thing from saying that they 

 possess the color-sense. 



