030 RADIATION BIOLOGY 



weak illumination this gives rise to complex behaviors, but in stronger 

 light orientation is much more accurate. The supra-esophageal ganglion 

 is not necessary for reactions to light or for photic orientation (Hess, 

 1921, 1924, 1925), but with ganglion and anterior end intact, light sensi- 

 tivity is higher and responses different in direction (positive to light with- 

 out ganglion or anterior end of body, negative to light when these are 

 intact). Segall (1933) also found reactions following intensity regardless 

 of area exposed by any given degree of extension, and hence no central 

 summation in the nervous system. Unteutsch (1937), on the other hand, 

 reported a definite relation between reaction time and area of worm 

 exposed to light, with increase in time following increased percentage of 

 total surface illuminated. This worker stated further that the maximum 

 sensitivity to shadowing is in the yellow region of the spectrum, whereas 

 that for general hght perception is in the blue; sensitivity to shadowing 

 was distributed uniformly, and that to illumination was greatest at the 

 two ends, with a maximum at the anterior end and a minimum at three- 

 quarters of the length toward the anus. From these data two separate 

 receptor sj^stems were postulated, with correlation between light and 

 shadow effects at a segmental ganglionic level rather than being mediated 

 centrally. No anatomical basis for these two types of receptor has been 

 reported. 



Hirudinea. Leech ocelli have been described extensively. They ap- 

 pear to be the chief specialized sensory organs and in the first five body seg- 

 ments appear to replace the lateral line organs. Each ocellus is approxi- 

 mately cylindrical, with its longitudinal axis at right angles to the body 

 surface. An outer layer of dark pigment surrounds a number of clear 

 refractive cells. The nerve fibers enter the organ at one side and con- 

 tinue up the axis to the several sensory cells. Whitman (1893) concluded 

 that they were modified tactile elements; Meryll (1894) distinguished 

 between the photosensory structures and those which were associated 

 and served the sense of touch. Apathy (1902) distinguished three phylo- 

 genetic steps in their specialization. Even the pattern of eye arrange- 

 ment is characteristic of the various genera. Behavior aspects have 

 been described by Bohn (1907), Herter (1929), and Schlueter (1933). 

 Response to light depends on the state of nutrition and involves both 

 the eyes and general dermal photosensitivity. Schlueter severed the 

 nerve connections to the five eyes on one side in Hirudo but produced no 

 asymmetry of response. Again the light reactions appeared not to be 

 mediated through the central nervous system as an image of the outside 

 world. Yet reactions to light intensity in each eye have been reported, 

 in terms of altered pigment distribution, in work by Stschegolew (1927) 

 and Wells (1932). These seeming discrepancies between behavior and 

 structural possibilities emphasize a point made by Parker (1922), i.e., 

 that the presence of photoreceptors does not mean that an animal can 



