THE NERVOUS SYSTEM 



191 



said to be the most primitive nervous epithelium in the body 

 (Fig. 124). 



Light Receptors. — Sensitivity to light is a characteristic of 

 protoplasm exhibited in its simplest form by organisms being 

 attracted or repelled by light. Phototaxis, as this reaction is 

 called, occurs even in forms that have no special organs for 

 perceiving light stimuli, such as many Protozoa and some 

 Metazoa, such as Hydra. Where special receptors exist, these 

 in lower forms are often merely mechanisms for distinguishing 



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Fig. 123. Fig. 124. 



Fig. 123. — Section of a taste bud, diagrammatic. Four taste cells are shown 

 with their outer ends converging toward the pit opening on the surface above. 

 The inner ends of the cells are in contact with nerves entering the base of the bud. 

 The pale cells within the bud are supporting cells. 



Fig. 124. — Section of human olfactory epithelium, diagrammatic. The slender 

 outer ends of the olfactory cells are ciliated. The process leaving the base of 

 each olfactory cell is a nerve fiber. 



between degrees of light. Such animals "see" only in the sense 

 that they are capable of distinguishing light from dark. An 

 example of such an eye is found in Planaria gonocephala. In 

 this form the paired eyes are located in the head region, one on 

 either side of the cephalic ganglion. Each eye consists of a cup- 

 shaped structure lined with pigment, into which project the 

 processes of 20 or more visual cells, each process being somewhat 

 thickened at its distal end to form a rhabdome (Fig. 125). 

 Proximally the visual cells are continued as fibers that form the 

 optic nerves extending to the cephalic ganglion. The eye is 

 sunk in the tissues and covered by a transparent epithelium, 

 continuous with the ectodermal body covering. Light passes 

 through the transparent epithelium and through the cell bodies 



