INVERTEBRATE PHOTORECEPTORS 637 



see. In many animals, as in jellyfishes, nerves from sense organs pass 

 directly to muscles or via ganglia to muscles. Hence activities due to 

 light or shadow correspond more closely to those of our own pupil control. 

 Higher development of nervous centers in relation to visual organs allows 

 a second function, informing the animal about its surroundings. This is 

 distinct from and superimposed upon the delivery of impulses along nerve 

 fibers that excite muscles to action. These are the light aspects of the 

 neurological statement made by Loeb (1894), when he pointed out that 

 there is no apparent parallelism between brain function in any species of 

 worm and its systematic position and that the independence of parts of 

 the body and of physiological processes (including photosensitivity) varies 

 from one to another without clear relation to phylogenetic bases. 



BRYOZOA 



Response to light by Pectinatella larvae and by Lophopus was reported 

 by Marcus (1925), but no special sensory mechanisms have been studied. 



MOLLUSCA 



Amphineura. The discovery of eyes here began with two papers by 

 Moseley (1884, 1885) reporting these structures on the outer surfaces of 

 the shells on exposed areas. Mostly these ocelli were oval or circular in 

 outline, from 65.0 by 42.5 n in Corephium to 0.73 mm in diameter in 

 Acanthopleura. Each has a highly refractive corneal lens surrounded by 

 a narrow zone of dark pigment, but both the presence of ocelli and the 

 manner of their arrangement are characteristic of the various genera of 

 chitons. Plate (1897, 1899) provided information about these structures 

 and distinguished clearly between the ocelli and two types of ''esthetes" — 

 macroesthetes, where several sensory cells were clustered below a cuticu- 

 lar thickening, and microesthetes, which are unicellular. Heath (1904) 

 traced ocellus formation in several genera and believed it improbable that 

 the ocelli were functional only in larval stages. Thus an adult Corephium 

 may have as many as 3000 ocelli on the anterior plate of the shell, with 

 perhaps 8500 as a total number for the whole organism. Behavior 

 studies, however, place no emphasis on these supposedly photosensory 

 structures. Crozier and Arey (1918), Arey and Crozier (1919), and 

 Crozier (1920a) found a gradual change in response to light, from nega- 

 tive to positive, as Chiton aged and grew larger. Areas bearing ocelli 

 were sensitive, but so too were the whole dorsal surface of the scaly 

 girdle and even the soft ventral surface of the foot. Sudden increase in 

 illumination of the ocelli failed to elicit responses, although similar treat- 

 ment of the girdle was effective. The whole dorsal surface was sensitive 

 to shading, and the activity of the animal depended on the steady illumi- 

 nation received there. Shadow reactions were obtained most strongly 



