VISION 483 



movement on the part of the fluid and particles stimulates 

 the sensory cells by rubbing against the hairs, and thus 

 produces sensation. Such organs are called otocysts, and 

 are to be compared with the vestibule and semicircular 

 canal structures of our ear rather than with the cochlea. 



In the vertebrates all, except the lowest fishes, possess 

 a structure similar to the inner ear of man located usually 

 in the head. The external ear, however, is a structure 

 peculiar to mammals. The cochlea and canals of the 

 human ear are supposed to have developed from the 

 simple vesicle of the lower forms. It is supposed, for 

 example, that the vestibule, with its utriculus and sacculus, 

 corresponds to the original vesicle from which the canals 

 and cochlea have arisen as outgrowths. The arrange- 

 ment of the sensory cells, and the presence of otoliths in 

 these parts, is very suggestive of the otocySt of the lower 

 form. 



Vision. — The simplest form of a visual organ consists 

 of a pigment spot overlying a sensory cell which is the 

 termination of a nerve fiber. The pigment spot absorbs 

 the light and converts it into a stimulus to the sensory 

 cell. Such spots are found on the disks of the jelly- 

 fishes and in the ray tips of a starfish. 



The next step is to cover the pigment spot with a trans- 

 parent thickening or lens. This lens by forming an 

 image permits the animal to distinguish form instead of 

 simply shapes of light and darkness as when the pigment 

 only is present. In the snail we have such simple eyes 

 or ocelli. Here the lens is in the form of a sphere covered 

 with a transparent cornea and its image is projected upon 

 a nervous network which we may consider as a simple 

 retina. Over this network is the pigment layer. In the 



