INVERTEBRATE PHOTORECEPTORS G61 



pared latency to shadow reaction with latency to mechanical stimulation 

 and concluded that, since the former extended from 0.50 to 0.85 sec, 

 whereas the latter extended from 0.11 to 0.25 sec, two different sensory 

 mechanisms must be involved. Later papers (Sarasin and Sarasin, 1885; 

 von Uexkuell, 1900) employed the Ceylon species of Diadema in terms of 

 light sensitivity and shadow reactions. Millott (1950, 1952, 1953), 

 describing a Jamaican species of this genus, indicated clearly that the 

 photosensitivity is dermal, with no specialized sensory structures found, 

 suggesting that diffuse branches of the radial nerves might contain neuro- 

 nal photoreceptors. A curious form of dark adaptation was reported in 

 this species in that melanin-containing chromatophores in the skin con- 

 centrate their pigment as light intensity decreases, allowing light to pass 

 between the chromatophores; hence a given stimulus is progressively 

 more efficient in eliciting a response. Local light stimulation of the 

 organism when in the pale dark-adapted condition also causes local 

 expansion of the pigment granules. The response in all cases is a muscu- 

 lar movement of the long poison-bearing spines toward any region in 

 which there is a sudden decrease in illumination. This may take the 

 form of a shadow falling on the animal or merely a 40-watt incandescent 

 lamp turned off from among a checkerboard of surrounding illuminating 

 100-, 60-, and 40-watt bulbs. 



Cowles (1911a), working on Toxopneustes, found reactions to light and 

 shadow in isolated pedicellariae, and these responses continued even when 

 the pedicellariae were disconnected from the body; he concluded that 

 "tonus centers" must lie in the tissues of the pedicellariae themselves. 

 This he contrasted with von Uexkuell's findings on Diadema, in which 

 severing the radial nerve connections to the spine base ended movements 

 in response to light. Millott supports von Uexkuell in that the spines of 

 Diadema are not photosensitive and the radial nerves are essential for 

 mediating the light responses. Holmes (1912), working with Arhacia, 

 reported similar movement of spines toward a spot of light thrown on 

 the body surface; this response and the contraction of tube feet in light 

 beams were not affected by cutting the nerve ring around the mouth, 

 but after such surgery the animal was unable to crawl away from light 

 into shadow (the normal reaction). Erection of spines in Arhacia as a 

 shadow reaction seems strictly comparable to that reported in Diadema, 

 but the response is much slower and less spectacular. Dubois (1913) 

 gave an almost unbelievable account of sea urchin response to light for 

 Strongylocentrotus, which commonly carries debris on its back by means 

 of tube feet and pedicellariae. If light was directional and strong, Dubois 

 found that these urchins picked up glass disks from the bottom of the 

 aquarium and held them in such positions as to give maximum shade. 

 Red glass was taken readily, but not green; circles and squares were 

 handled indiscriminately; when the light directions were altered, the 



