320 JELLY-FISH, STAR-FISH, AND SEA-URCHINS. 



coiit, will be found congfue^'.ated in the narrow slit 

 of light. The source we used was diffused day- 

 light, which was admitted through two sheets of 

 glass, so tliat the thciTiial rays might be considered 

 practically excluded. The intensit>j of the light 

 which the Echinoderms are able to perceive may be 

 very feeble indeed ; for in our first experiments we 

 boarded up the face of the tank with ordinary pine- 

 wood, in order to exclude the light over all parts 

 of the tank except at one narrow slit between two 

 of the boards. On takino- down the boards we 

 found, indeed, the majority of tlie specimens in or 

 near the slit of light; but we also found a number 

 of other specimens gatliering all the way along the 

 glass face of the tank that was immediately behind 

 the pine-boards. On repeating the experiment 

 with blackened boards, this was never found to be 

 the case ; so there can be no doubt that in the first 

 experiments the animals were attracted by the 

 faint glimmer of the white boards, as illuminated 

 by the veiy small amount of light scattered from 

 the narrow slit through a tank, all the other sides 

 of which were black slate. Indeed, towards the 

 end of tlie tank, where some of the specimens were 

 found, so feeble must have been the intensity of 

 this glimmer, that we doubt whether even human 

 eyes could have discerned it very distinctly. Owing 

 to the prisms at our command not having sufficient 

 dispersive power for the experiments, and not wish- 

 ing to rely on the uncertain method of employing 

 coloured glass, we were unable to ascertain how the 

 Echinoderms might be affected by different rays. 



