628 RADIATION BIOLOGY 



and that light focused toward the periphery of each pigment cup was only 

 one-ninth as effective as that reaching the center of the sensory area. 



In flagellates without stigmata, such as Peranema, Shettles (1937) and 

 Shortess (1942) concluded that the whole cell was photosensitive in a 

 gradient from least in the posterior end to most in the flagellum. Even 

 detached flagella responded to increased illumination by bending, 

 although no recovery seemed possible except in intact specimens. A 

 reasonable spectral-sensitivity curve was obtained from these studies 

 (Shettles, 1937) ; dark and light adaptation indicated definite change in 

 the photosensitive substance. 



COELENTERATA 



The earliest comments on the light relations of invertebrates concerned 

 Hydra (Trembley, 1744). In this animal, with no visible photoreceptors, 

 Wilson (1891) reported movements into parts of an aquarium shaded 

 with blue glass; he claimed that this was independent of intensity for 

 both brown and green Hydra exposed to several colors simultaneously. 

 Haug (1933) found no correlation with wave length, but a shadow reac- 

 tion seemed definite at the oral end. Both Haug (1933) and Schluensen 

 (1935) investigated the ability of Hydra to distinguish between light 

 directed toward it from various directions, singly or in multiples. Lack 

 of a spectral characteristic seems peculiar, however, since the mechanism 

 might be expected to be similar to that investigated in the hydroid 

 Eudendrium by Loeb and Ewald (1914) and Loeb and Wasteneys (1917), 

 in which good correlation was found between prediction based on the 

 Bunsen-Roscoe product law of intensity and exposure time and activity 

 observed in orientation of the polyps to directed illumination. 



Driesch (1890) reported on the accuracy with which the hydroid 

 Sertularella was oriented with respect to lateral illumination. Bohn 

 (1906) suggested that the anemone Actinoloha reached a position with 

 the oral pole toward the light by a series of trial-and-error movements. 

 Moore (1924, 1927), however, found remarkably precise orientation in 

 Cerianthiis, with the angle turned proportional to the logarithm of inten- 

 sity, and a spectral sensitivity having a maximum between 510 and 

 570 m^. McClendon (1910), Schmid (1911), and Gee (1913) with other 

 anemones described the tentacles as expanded in light, retracted in dark. 

 McClendon explained the activities and position of Cradactis in rock 

 crevices as due to a negative response to light in the foot combined with 

 a positive response in the tentacles. Gee offered the hypothesis that in 

 Cribrina the light reaction in the tentacles was due to photosynthetic 

 products from symbiotic algae in this anemone. Since the phenomenon 

 is widespread and the symbionts are not, general sensitivity without 

 organized photoreceptors seems a more adequate explanation. 



Ocelli associated with the tentacles of medusae have been illustrated 



