660 RADIATION BIOLOGY 



experimented with Asterias stimulated simultaneously by two or more 

 separate light sources and found a variety of responses in the same indi- 

 vidual. Three different patterns emerged: (1) direct movement toward 

 one source, (2) movement with no clear relation to any source, and (3) 

 movement on the resultant between two sources as far as a critical point. 

 This point having been reached, the animal either (1) angled obliquely 

 toward one source, (2) was already on the line joining the sources and 

 there made a 90° turn toward one source, or (3) had gone beyond the 

 line joining the sources and thence made an acute change in direction of 

 movement to move to one source. 



The stereotropic orientation of starfish tube feet has been used as a 

 test in connection with light responses. Thus Astropecteri illuminated 

 from below rather than from above failed to right itself unless the raj^ 

 tips with their ocelli had been removed; this operation allowed the right- 

 ing response at once (Wolf, 1925). Asterina showed positive response 

 to light when the aboral side was up but negative when inverted (Kalmus, 

 1929). The significance of these findings in terms of photoreceptors is 

 far from clear. Probably van Weel's conclusion (1935) is most repre- 

 sentative, namely, that, light sensitivity is characteristic of the ocelli, 

 the terminal tentacles, the ambulacral tube feet, and the general body 

 surface; that the entire skin area is able to inform the animal of shadows; 

 and that some degree of directionality is interpreted by the central 

 nervous system, leading to kinetic responses with definite orientation. 



Detailed studies of the nervous connections in the starfish Marthasterias 

 by Smith (1937, 1947) have yielded more information on the dermal 

 photoreceptors. These seemed to be of a single kind, each a bipolar 

 spindle-shaped cell 5-10 ix long, 1-1.5 ju in diameter, extending at either 

 end into a long slender filament, and reaching the remarkable abundance 

 of 70,000 per square millimeter. At the moment, information available 

 on the nervous system does not indicate how far discrimination between 

 physiologically different photoreceptors can be differentiated to inform 

 the animal of its surroundings. 



Ophiuroidea. Ophiuroids have been investigated, and their sensitivity 

 seems to be dermal, general, and with no known photoreceptors. Cowles 

 (1910) reported that Ophiocoma reacted negatively to brightly lighted 

 fields unless some other factor changed the response. Thus these animals 

 would not remain in a shaded portion of the aquarium unless they were in 

 contact with the shading object. To put it another way, the shadow 

 produced in a cavity has a different stimulating effect than a shadow 

 alone. Cowles concluded that ophiuroids react to dark vertical walls 

 even when they cast no shadow; this would indicate directionality com- 

 parable to that reported in asteroids. 



Echinoidea. The earliest photosensory studies of echinoids appear to 

 be those of von ITexkuell (1897) on Cenirostephanus, in which he com- 



