640 RADIATION BIOLOGY # 



tissue — algae that these clams engulf as food through the action of 

 amebocytes as a substitute for filter feeding. Thus the hyaline bodies 

 are not photoreceptors but are part of the nutritional system. 



Gastropoda. Among the gastropods a considerable range of photo- 

 sensory structures is known. The limpets Patella and Acmaea have a 

 simple pit lined with retinula cells that secrete a thin hyaline covering 

 conforming to the inner contours of the pit (Fig. 14-6). The abalone 

 Haliotis has a far deeper pit, and the secretion not only fills it but expands 

 into a convex light-concentrating structure on the body surface. Key- 

 hole limpets such as Fissurella have a closed sac just below the surface, 

 with no secretion — hence an "outer" and an "inner" cornea. In many 

 other snails the ocellus is like that of the whelk Murex in having a large 

 lens eccentrically placed in the secretion filling the optic space, so that 

 the retina is somewhat remote from the lens, and an image of sorts may 

 be formed. Land snails such as Helix frequently have such large lenses 

 that the secreted hyaline spacing is reduced to a thin film and no satis- 

 factory image seems possible. These comparative aspects (Fig. 14-6) 

 have been furnished by various workers. Willem (1891, 1892) concluded 

 that the tactile sense of terrestrial forms was so well developed that they 

 paid httle attention to light and could distinguish form only at 1-2 mm 

 and voluminous objects at no more than 1 cm. He further summarized 

 his findings by indicating that aquatic pulmonates have no distinct 

 vision at any distance and that all the pulmonates chiefly depended on 

 dermal photosensitivity. Yung (1913) claimed that Helix eyes were 

 blind, because he could demonstrate a lack of continuity in the optic 

 nerve — a gap in it between eye and cephalic ganglion. Others have 

 extended the generalization regarding dermal photosensitivity to various 

 slugs {Arion and Limax) and snails. Yet physiological investigations of 

 these same animals indicate circus movements when one ocellus is stimu- 

 lated, as in nondirectional light with one eyestalk tip (and ocellus) 

 removed. Crozier and Libby (1925) reported a temporary loss of light 

 responses in Limax after meals of cooked potato but not after raw-potato 

 ingestion. Mitsukuri (1901) found that Littorina responded to light in 

 a different sense depending on whether the animal was wet or dry. Bohn 

 (1904a, b, 1905a, b) investigated these reversals of response; Dimon (1905) 

 extended the same principle to the mud snail Nassa. Fraenkel (1927) 

 noted that, when Littorina were inverted, they responded positively to 

 light; when upright, negatively. The complex alterations of response 

 are believed to explain migrations of the animals with respect to land 

 and tidal movements. 



A far higher degree of visual response was seen in the opisthobranch 

 Elysia, where a visual field could be investigated for the pigment cup 

 at the base of the tentacle (ibid.). Dijkgraaf (1935) measured a latency 

 of 0.5 sec in the contraction of the marine snail Aplysia stimulated by 



