82 



ZOOLOGY 



there are also external vibratile hairs and vibrating membranes which are auditory 

 (Fig. 42). 



112. Sight. There are three distinct facts to be noted with respect to visual 

 sensation in the higher forms of animals: the perception of light, the perception of 

 color (i. e., light of different wave-frequency) and the formation of images of ex- 

 ternal objects. It has already been seen (21) that protoplasm is sensitive and 

 responsive to light without any special organs. The simplest visual organs found 

 in multicellular animals consist merely of epithelial cells containing pigment in 



FIG. 43. 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 depression (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, Lizzta); F, retinal cells more highly magnified, ep., 

 epidermis;/, 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? 



which changes are wrought apparently by the action of light (Fig. 43, a). These 

 changes affect the nerve fibres associated with the pigment cells and thus the 

 central nervous organ. Such eyes are capable only of giving knowledge of the 

 intensity or, if properly constructed, of intensity and direction of the light and do 

 not form an image of external objects. There are several types of image-forming 

 eyes in the animal kingdom. The most familiar of these is the "camera eye" of 

 vertebrates, so called because it illustrates the principles made use of in the 



