444 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



Such eye- spots are possessed by a number of the 

 lower Invertebrata. They are, perhaps, found at their 

 simplest condition in a starfish, where they occupy the 

 ends of the arms ; and these ends are, by a muscle run- 

 ning along the upper surface of the arm, turned upwards 

 so as to be exposed to the light. There are here (Fig. 

 190) a number of eye-spots, each of which is made 

 up of several epithelial cells j these undergo a 



Fig. 190. Four separate Eye-spots of a Starfish, showing the invaginated 

 Epithelial Cells and the Central Cavity ; below is the plexus of 

 Nerve Fibres. (After Hamann.) 



certain amount of invagination, and give rise to a 

 central cavity in their midst ; with these cells nerve 

 fibres become connected, and in their substance a red 

 pigment is deposited. Here, then, we have nothing 

 more than a number of epithelial sensory cells, distin- 

 guished by the possession of pigment ; the cuticle, it 

 will be observed, is not thickened into a convex cornea, 

 and there is no reason to suppose that the fluid in the 

 central cavity has any refractive action on the rays of 

 light. 



The Medusae, or such of them as have eyes 

 present us with a condition which is a little in ad- 

 vance of what obtains in the starfish ; for, speaking 



