662 RADIATION BIOLOGY 



urchin reoriented its ]x)dy or the disks; and if a disk had a central trans- 

 parent spot, the animal used another disk to shade the "hole" in its 

 glass umbrella! 



Holothuroidea. Hamann (1883b) illustrated what he believed to be 

 light-sensitive eyespots in a sea cucumber. Clark (1898) traced the 

 development of these sensory areas until they became red spots at the 

 base of the tentacles. He found the arms to be photokinetic, respond- 

 ing negatively to a decrease in light intensity but not perceptibly to an 

 increase; the entire body surface appeared to be sensitive to light. Olm- 

 sted (1917) was unable to establish whether the eyespots described had 

 any light-sensitive function, though the whole body of Synaptida was 

 photoreceptive. Even heads of this genus, severed just back of the tenta- 

 cles, moved away from light; strong light on the headless bodies caused 

 them too to turn away. Removal of tentacles (with eyespots) followed 

 by unilateral illumination led to turning of the body in a direct line away 

 from the light. Amputated heads lit by a spot of light, however, did not 

 draw in the tentacles, as was characteristic of intact animals under such 

 stimulation. Crozier investigated holothurian behavior in light (1914, 

 1917, 1920a,b) and noted that Holothuria moved with definite polarity 

 (mouth anteriorly), whereas Thyone moved away from light like an 

 echinoid— with any angle of the body in advance. Again the whole sur- 

 face proved sensitive to light, with stimulation depending on the amount 

 of light falling on the surface and not on the angle of incidence. Even 

 isolated portions of the skin reacted to continuous light, thus indicating 

 that light itself, rather than change in intensity, furnished the stimulus. 



HEMICHORDATA 



Dermal photosensitivity of Balanoglossus and its relation to the nervous 

 system were investigated by Crozier (1917). Hess (1931, 1936, 1938a) 

 found no specific photosensory organs in adults of Ptychodera and Balano- 

 glossus but found that distribution of dermal photosensitivity corre- 

 sponded to the distribution of neuronal photoreceptors below the skin. 

 Hilton (1923) reported a possible eyespot at the tip of the preoral lobe in 

 Cephalodiscus and Rhahdopleura. Larval hemichordates apparently bear 

 eyespots with lenses (Spengel, 1893; Stiasny, 1914). 



CHORDATA 



Among members of the Urochorda, photosensory structures of ocellar 

 type have been described in the free-swimming pelagic colonies of Salpa 

 and related genera; no information on the use of these ocelli has been 

 published. Grave (1920) and Mast (1921) studied the ocelli and light 

 reactions of larval Amaroucium and found photosensitivity until the time 

 of metamorphosis. Each ocellus consists of a series of three lenses, a 

 cup-shaped mass of pigment granules, and a group of retinula cells. 



