PARKER. — THE SENSORY REACTIONS OF AMPHIOXUS. 4-17 



of the eye-cups are directed ventrally and the animal usually rests in 

 the sand obliquely with the ventral side uppermost. Thus the ma- 

 jority of the eye-cups are in a position to receive effective stimulation. 

 If we imagine the body of amphioxus to be increased in muscular 

 strength, etc., whereby it would approach more nearly the condition in 

 the fishes and would consequently add much to its thickness, it follows 

 that the posterior portion would become less transparent and the pho- 

 toreceptors of the anterior end would be the only ones left in position 

 for effective stimulation. With the development of the mouth cavity, 

 the gills, etc., the source of light for the anterior photoreceptors would 

 become chiefly lateral and dorsal, and their orientation would doubtless 

 conform to this plan of illumination. If in accordance with this scheme 

 each eye-cup assumed the best possible orientation, it would lie with 

 its open end directed laterally and perhaps somewhat dorsally, i. e., the 

 contained sense cell would be oriented with its sensory end away from 

 the light and its nervous end towards this stimulus. With the dis- 

 appearance of the surrounding pigment cells as the cluster of photo- 

 receptors became a single retina, these elements would be oriented as 

 the rods and cones are. It is in this way, I believe, that the rods and 

 cones of the vertebrate eye have become inverted, rather than that the 

 inversion is inherited from a condition on the external surface of the 

 body. 



Not only may the rod- and cone-cells be thus oriented at the begin- 

 ning, but it seems to me that their subsequent relations to the surround- 

 ing parts tend to keep them so. The chief factor in this respect is the 

 supply of materials necessary for their activity. Directed as they are 

 away from the central dioptric part of the eye, their sensory ends, 

 which are the parts most quickly exhausted by activity, are turned 

 toward the chief blood-supply, the choroid layer of the eye, and are, 

 therefore, in a most advantageous position to receive new materials for 

 metabolism. That important substances reach them from this side is 

 seen in the fact, well attested by experiment, that if the retinal pigment 

 layer is removed from a live retina, the regeneration of the visual pur- 

 ple in the rods is much retarded, if not completely stopped, though 

 simply placing the layer back again upon the retina will cause this 

 process to be resumed. Thus the inverted position of the rod- and 

 cone-cells is the one best adapted to keep their most easily exhausted 

 parts nearest the supply of materials necessary for their activities and 

 still hold them open to access to light. This factor is doubtless one 

 that has tended to retain the rod- and cone-cells in their inverted 

 positions. 



The condition of light receptors in amphioxus lends no support to 



