INVERTEBRATE PHOTORECEPTORS G63 



Larval Botrijllus has a similar ocellus (Woodbridge, 1924). Grave and 

 Riley (1935) described the embryonic development of these photosensory 

 structures. The so-called "ocelli" of adult Ascidia have proved to be 

 insensitive to light stimulation (Hecht, 1918a), but neuronal photo- 

 receptors within the oral siphon in the general region of the tentacles 

 (1.0-1.5 cm below the "ocelli") are sensitive and cause immediate retrac- 

 tion of the siphon if illuminated. In other work Hecht (1918b, 1921c, 

 1926, 1927) studied photochemistry in terms of light responses in Ciona. 

 The "eyespot" at the anterior end of Amphioxus has proved to have 

 no photosensory function. Along the nerve cord, however, are true eye- 

 spots that provide a sensitivity to light throughout the length of the 

 animal. The degree of photosensitivity is roughly proportional to the 

 population of these pigment-backed sensory cells. The most reliable 

 accounts are those of Hesse (1898) and Parker (1906). Another possible 

 type of photoreceptor has been found in Amphioxus, located dorsally 

 along the nerve cord (Joseph, 1928). Known now as "Joseph's cells," 

 these structures are unicellular and lack obvious pigment but otherwise 

 correspond in detailed anatomy to the true eyespots. 



SUMMARY 



Throughout invertebrate phyla general photosensitivity is widespread. 

 In many phyla specialized photoreceptors are added to the dermal recep- 

 tor system, but the degree of organization of the specialized organs is not 

 invariably correlated with obvious value in living habits. In many 

 instances the nervous system appears unable to utilize fully the acuity 

 provided by the dioptric apparatus. Adaptation to a range of light 

 intensities, light reactions, and shadow responses seem to be functions 

 primarily of neuronal photoreceptors and only secondarily of organized 

 eyes. Photosensitivity of ganglionic nervous tissue has been found. It 

 may be suspected of being a primitive characteristic. 



Development of a functional lens, with or without an outer cornea, 

 has occurred in annelids, mollusks, and arthropods. No homologies seem 

 indicated. In each of these phyla are specialized types in which the eye 

 can be accommodated. Compound eyespots, compound ocelli, and com- 

 pound eyes have arisen among annelids, mollusks, and arthropods — ^in 

 each instance with numerous basic differences in organization. In all 

 an eye with increased resolving power has been achieved, permitting the 

 animal to detect more details of its environment than is possible through 

 dermal photosensitivity or any type of eyespot or eye less complex than a 

 camera-style organ. 



THE PHOTOSENSORY MECHANISM 



Electrical records of action-potential changes in invertebrate photo- 

 receptors were obtained first by Dewar and M'Kendrick (1873) with 



