130 



THE EYE IN EVOLUTION 



is true, merely records the sensation of light, but the summation of 

 all their individual sensations will give an elementary mosaic or pattern 

 of light and shade with a consequent impression of the external world. 

 So long as the component cells of the group retain their individuality 

 and act independently of each other, they may be considered to form 

 a " simple " eye. The simple eye or ocellus (dim. of oculus, eye) 

 may therefore be defined as a single light-sensitive cell or group of such 

 cells acting ivithout functional association. 



Leucosolenia 



THE UNICELLULAR EYE 



Single cells which are responsive to light (" cellules visuelles " of 

 Apathy, 1897 ; " Photores " of Beer, 1901) were first adequatel}^ 

 described by Richard Hesse (1896) as occurring in the epidermal layer 

 of worms ; he called them " Sehzellen ", but since in many cases they 

 appreciate the presence of light for the purposes of jDliototactic reactions 

 and are probably not associated with a visual sense as the term is 

 generally understood, we shall call them light-sensitive cells. 

 Shortly thereafter they were found in a large number of animals, 

 sometimes scattered about indiscriminately but usually aggregated in 

 those regions where they are of most importance to the organism. Thus 

 in clams they are confined to the siphon, in some shell-fish are arranged 

 like a coronet around the edge of the mantle, and in annelid worms they 

 are concentrated at the two extremities, particularly the anterior.^ 



Unicellular eyes may assume either of the two forms of light- 

 sensitive cell we have already discussed — the bipolar form with a 

 specialized sensory termination or the apolar 

 form characterized by an intracellular organelle. 



SINGLE BIPOLAR LIGHT-SENSITIVE CELLS are 



usually provided with a ciliate or brush-like border 

 and are associated with pigment, usually placed as 

 a cap around the light-sensitive end of the cell. 

 They are seen in the unicellular photoreceptors of 

 the larvse of certain sponges {Leucosolenia, 

 Minchin, 1896) and in Rotifers, but occur most 

 typically among worms. Examples of this are 

 the light-sensitive cells of Tristomum papillosum, 

 a Trematode parasitic on marine fishes (Fig. 87), or 

 in Polyophthalmus pictus, a sedentary Polychsete 

 which abounds in the Bay of Naples (Hesse, 1899- 

 1908). 



eye 



is interesting that the most primitive "cerebral " 

 ordates, seen in the neural tube of Amphioxus^ 



190. 



p. 230. 



Fig. 87. — Unicellu- 

 lar Eye of the 

 Trematode Worm, 

 Tristomum papil- 

 losum. 



The cell is provi- 

 ded with a crenated 

 border and piginent 

 mantle (after Hesse). 



