626 



HANDBOOK OF PHYSIOLOGY 



NEUROPHYSIOLOGY I 



veloped its pigmentation and while light can pene- 

 trate more readily into the viscera. 



Peripheral Photosensitivity 



The degree to which light responses can be localized 

 into effective behavior patterns without structural 

 specialization of photoreceptors is certainly shown in 

 the echinoid echinoderms. With the Mediterranean 

 urchin Centrosteplianiis, von Uxkiill found fairly rapid 

 adjustments in orientation of the spines according to 

 the direction from which the body was shadowed 

 (275). A more detailed study of the Caribbean 

 Diadema by Millott (i 88-1 91) confirmed von Uxkiill's 

 findings on behavior and established the fact that no 

 special photoreceptor cells are present at the ends of 

 the twigs from the radial nerves where these enter 

 the dermis. Yet the entire body surface appears 

 photosensitive, only the spines themselves lacking this 

 type of irritability. The radial nerve must be intact 

 for the responses to follow local stimulation. Even a 

 kind of dark adaptation is present through concentra- 

 tion of pigment in dermal chroma tophores, permit- 

 ting more light to penetrate to the level in the skin at 

 which the nerve twigs lie. 



With another Caribbean urchin Millott was able 

 to duplicate some of the spectacular findings of Dubois 

 (59) on European Strongylocentrotus. Both echinoids 

 have the habit of partially covering the aboral sur- 

 face with debris picked up from the adjacent bottom. 

 The Caribbean Lvtechinus inhabits the coral reefs and 

 boulder-strewn beaches to the limit of wave action 

 at low tide and appears to use bits of coral as ballast 

 in this buffetted zone; but if a narrow beam of light 

 is directed on any portion of the aboral surface, the 

 urchin transfers these opaque objects (or any bits of 

 seaweed within reach of tube feet and pedicellariae) 

 into the path of the beam, using them as a parasol. 

 Urchins at greater depths, where the light is less in- 

 tense, seem to carry debris only while sunlight is 

 reaching them on the bottom (191). 



Perhaps the best example of an extreme sensitivity 

 to shadows was found by Millott with Diadema. When 

 a single urchin was placed in a finger bowl of sea 

 water under a checkerboard of electric lamps, it 

 would rapidly point many spines in the direction of 

 any single lamp in the pattern when this one was 

 temporarily turned off. Identification of the direction 

 in which so minor a change occurred must be medi- 

 ated through an inherent polarity with maximum 

 sensitivity to light reaching the body surface at right 



angles, as well as through the general roundness such 

 as Nagel postulated (201) (fig. 2 right~). 



A general photosensitivity with less striking re- 

 sponses has Ijeen demonstrated in other echinoderms: 

 in the entire aboral surface of the sessile (and swim- 

 ming) crinoid Antedon (163); over the whole body of 

 the holothurians Synaptula (203) and Holothuria (41); 

 o\er the aboral surface of asteroids from which the 

 ocellatc tips of the arms had been removed (258, 297); 

 and in the ophiuroid Ophiocoma (37). Crozier found 

 a difference (41) between the behavior of Holothuria 

 and Thyone in that the latter holothurian mo\cd away 

 from a light source as an echinoid might — any angle 

 of the body in advance. Holothuria, by contrast, 

 showed a functional polarity, swinging around until 

 the mouth was farthest from the stimulating light 

 before moving off in this orientation. 



Responses to light where no receptors seemed 

 specialized toward sensiti\ity to radiations have been 

 reported in blinded and intact members of many 

 phyla: in the hydrozoan medusa Gonionemus (200); in 

 luminescent ctenophores Beroe (122) and Mnemiopsis 

 (197); in blind turbellarians (165); in blind rotifers 

 (263, 264); in nematodes (104); in oligochaetes (no, 

 III, 114, 161) with identification of neuronal photo- 

 receptors in which photo.sensitivity was localized; in 

 the polychaete Mercierella (228); in the leech Hirudo 

 (234); in bryozoans, both as larvae (Pectinatella) and 

 adults QLophopus'), through kinetic responses of nega- 



FIG. 2. Curvature of the body surface can provide an animal 

 having general photosensitivity with a means for identifying 

 the direction from which a light stimulus comes. Neither lamp 

 A nor shadow B have a directional significance for a flat photo- 

 sensitive tissue (/f/O; but in a cylindrical or spherical organism 

 (jigtiO quite different cells are illuminated by the two sources, 

 A and B. [After Nagel; from Milne & Milne (193).] 



