6o Journal of Comparative Neurology. 



best suited to enable these worms to receive luminous impress- 

 ions from above and from either side — as would be necessary 

 when they leave their burrows. The greater number of these 

 organs on the prostomium, palps, and peristome and also their 

 presence on the ventral surface of the peristome would be a 

 great protection when the animal simply thrusts its head from 

 its burrow. Since Nereis possesses true eyes, these spiral or- 

 gans probably serve, not for the perception of distinct images, 

 but of difference in intensity of light. Nagel ('96) has shown 

 that animals which lie buried in the sand with only a small por- 

 tion of their body exposed have this portion supplied 

 with ocular organs. Experiments show that, in such cases, 

 this part is extremely sensitive to differences in intensity 

 of light and is quickly withdrawn when a faint shadow is 

 cast upon it. Nagel therefore concluded that these organs 

 give warning of the approach of enemies by the perception of 

 their shadows. In the absence of direct experiments on the 

 spiral organs of Nereis it may be concluded that the general 

 explanation given by Nagel is also applicable to Nereis — 

 that the spiral organs are ocular organs which have for their 

 function the perception of differences in intensity of light. 



VI. Paired Cephalic Sense- Organs. 



A lack of the necessary material and time has prevented 

 me from making a thorough study of the paired sense-organs 

 found in the head of Nereis virens. I can, therefore, merely 

 give the few facts of interest which have come under my obser- 

 vation during the foregoing study. 



The two pairs of eyes have been described in N. virens 

 by Andrews ('92) and I can add but little to this description. 

 In some of my methylene blue preparations, the retinal cells 

 were well stained and it could be distinctly seen that, in many 

 cases, the peripheral protoplasmic ends of the retinal cells them- 

 selves passed through the pigment layer and appeared within 

 the cup. The lens was shrunken and connected with the ret- 

 inal rods by strands as described by Andrews. When the apex 

 of a retinal cell appeared within the cup, it, too, was connected 



