CHAPTER VII 



THE SYSTEMATIC ANATOMY 

 OF INVERTEBRATE EYES 



From the morphological point of view we have seen that the 

 visual organs of Invertebrates show an astonishing range in structure, 

 varying in complexity from the simple eye-spot or the single visual cell 

 to the elaborate organs characteristic of Cephalopods or Insects ; from 

 the functional point of view the variation is equally great, evolving 

 from a primitive and j^erhaps undifferentiated sentiency which may 

 influence metabolic and motorial reactions, to the capacity to form 

 elaborate images whereby intensity, hue, form and spatial relationships 

 can be differentiated with sufficient exactitude and appreciation to 

 determine behaviour. The curious thing, however, is that in their 

 distribution the eyes of Invertebrates form no series of contiguity and 

 succession. Without obvious phylogenetic sequence, their occurrence 

 seems haphazard ; analogous photoreceptors appear in unrelated 

 species, an elaborate organ in a primitive species ^ or an elementary 

 structure high in the evolutionary scale, ^ and the same animal may be 

 provided with two different mechanisms with different spectral 

 sensitivities subserving different types of behaviour. 



A striking example of this is seen in the flat-worm, Planaria lugubris, which 

 has both positive and negative photo -reactions (Viaud and Medioni, 1949) ; if 

 this aniinal is bisected the photo -positive reactions appear in the posterior 

 segment before the nerves regenerate suggesting that these responses are due to 

 dermal sensitivity, while it has been shown that the photo -negative reactions 

 are due to the eyes ; photokinesis is dependent on the skin, positional orientation 

 to light on the eyes. In the earthworm, Lurnbricus terrestris, on the other hand, 

 the photo-negative reactions in bright light are controlled by the head -ganglion, 

 while the photo-jDOsitive reactions in dim light are nnediated by ; the ventral 

 cord ; the two activities are mutually antagonistic but normally the cephalic 

 mechanism is dominant (Prosser, 1934). Again, the possession of both ocelli 

 and compound eyes by many insects, the first sometimes reacting to polarized 

 light and orientative in function, and the second to ordinary light as well and 

 also subserving form vision, is an example of two mechanisms which are 

 supplementary in function and not antagonistic (Wellington, 1953). 



We shall now discuss the occurrence of these organs in the inverte- 

 brate phyla, referring back to the previous chapter for a description of 

 their ii: nute structure. 



^ Such as the complex eye of the jelly-fish, Charybdea (p. 183). 

 Such as the simple eyes of Insects (p. 224). 



