PHOTOSENSITIVITY IN INVERTEBRATES 



627 



live sign (179); in various pelecypods (201) but 

 particularly Mya (94, 96, 148, 166); in the gastropod 

 Helix on the basis of photosensitivity in spite of a gap 

 found in the optic nerve from the tentacular eye 

 (306); in the hemichordate Dolichoglossus (39, 11 1, 

 112); and in the adult urochordates Ascidia (93) and 

 Ciona C96). 



Photoreceptors in the skin of a soft-bodied animal, 

 such as an oligochaete, vary greatly in degree of ex- 

 posure depending on the extension and contraction 

 of the superficial tissues in locomotion. In Perichaeta, 

 Harper (85) found that a negative response might be 

 shown to low-intensity illumination w-hen the worm 

 was tested while extended, yet the same test applied 

 while the worm was contracted might lead to a posi- 

 tive response. 



Quite a few eyeless invertebrates (particularly 

 hydroid coelentcrates and turbellarians) react posi- 

 tively to sunlight without giving proof that they are 

 themselves photosensitive. These organisms harbor 

 mutualistic algae (green 'zoochlorellae' or brownish 

 green 'zooxanthellae') which carry on photosynthesis 

 when illuminated. A response to chemical changes 

 accompanying photosynthesis could account for the 

 behavior of the animal partner in many instances. 

 Even when the invertebrate possesses eyes as well as 

 mutualistic algae, the role of vision in photic re- 

 sponses becomes suspect until pro\ed definitely. 



Translucent bodies which stud the exposable por- 

 tion of the mantle in the giant clam Tridacna were 

 described as eyes until Yonge (305) cleared up the 

 misunderstanding. He found that these structures are, 

 instead, an adaptation permitting daylight to reach 

 deep levels of the mantle tissue where large numbers 

 of mutualistic algae grow. Tridacna appears to depend 

 for food primarily upon the success of the enclosed 

 algae. It raises the plants in mantle greenhouses, and 

 the supposed eyes are merely illuminators in the roof! 



Photosensitivity Mediated Thrmigh Unicellular Eyespots 



Addition of a cup of opaque pigment beside a 

 photosensory cell seems but a small step in evolution 

 but still a move toward development of an eye. This 

 addition permits the receptor to be more definite as 

 to the direction from which a stimulating light comes 

 than when its own greater sensitivity to radiation 

 passing along its axis is the sole means for differentia- 

 tion. When photosensory inechanisms consist of a 

 single receptor cell and an associated pigment mass, 

 the term eyespot is useful — although earlier authors 



have used the word far more loosely. Often a lens is 

 associated with an eyespot, providing still more dis- 

 criminatory possibilities and perhaps increasing the 

 structure's sensitivity by gathering in more light. 



Eyespots are present in such turbellarians as Pro- 

 rhynchus (^14.4) and in a number of parasitic trematodes, 

 particularly at various larval stages of digenetic 

 forms. Some trematode cercaria possess them; so does 

 the miracidium of Fasciola. Instances of apparent de- 

 generation have been identified (70), although no 

 evidence has been given that would indicate a cor- 

 responding loss of sensitivity. In many nemertineans, 

 one or more pairs of eyespots are present (123), but 

 whether negative responses to radiation found in 

 Lineus (194, 195) depend upon functional eyespots 

 has not been proved. By ingenious experiments, 

 Viaud has been able to distinguish between photo- 

 sensitivity in rotifers mediated by their eyespots and 

 those elicited on the basis of a general sensitivity 

 (260-264). Eyespots are the only photoreceptors 

 identified in archiannelids (125) and some poly- 

 chaetes (i 17, 237). They are characteristically present 

 in tardigrades (43) and larval hemichordates (243, 

 245). They are scattered along the nerve cord of 

 cephalochordates (116, 141, 204), and appear to be 

 the sole mechanism allowing response to radiations 

 (204); possibly degenerate eyespots, devoid of pig- 

 ment cups, were described by Joseph (141). 



Pliotosensitii'ity Mediated Through Multicellular Eyes 



COMPOUND EYESPOTS. Another small step toward ef- 

 fective vision consists of the grouping together of 

 unicellular eyespots, forming them into an organized 

 cluster with radially divergent axes. Structures of this 

 kind — compound eyespots — have been reported in but 

 three groups of organisms with no indication that 

 they are part of an evolutionary sequence. 



Both solitary and compound eyespots project from 

 the mantle margin of pelecypods in the genera Area 

 (fig. 6) and Pectunculus. In A. noae a specimen 8.5 

 cm long had 235 of these sensory clusters. Neither 

 Patten (206) nor Kiipfer (159), however, indicated 

 the degree to which the compound eyespots were 

 used in a visual way. 



The annelids Potamilla and Branchiomma (fig. 6) 

 bear compound eyespots on the main stems of the 

 cephalic branchiae (3, 27). Each sensory unit is 

 isolated from its neighbors by pigment cells. Yet the 

 known reactions of these polychaete worms seem no 



