THE COMPOUND EYE 



163 



lie directly beneath them. The retinule contains two types of cell. The main 

 mass is made up of about a dozen elongated sensory cells grouped round central 

 rhabdomes, their prolongations giving rise to fibres which mingle in a plexus 

 before they emerge to run proximally in the optic nerve (Fig. 144). In each 

 retinule there is also one eccentric cell the axon of which travels down the central 

 rhabdome and along the nerve ; it would seem probable that this forms the 

 conducting element for nerve imiDulses set up by stimulation of the retinular 

 cells (Hartline et al., 1953 ; Waterman and Wiersma, 1954). ^ A white pigmented 

 strvicture, the rudimentary eye, lies behind the posterior margin of the compound 

 eye and sends a third type of large nerve fibre into the optic nerve (Waterman, 

 1950 ; Waterman and Enami, 1953). 



THE COMPOUND EYES OF CRUSTACEANS 



Crustaceans show two types of compound eyes — a relatively 

 primitive type associated with the smaller siDecies and a well formed 

 type associated with the larger (crayfish, lobster, crab, etc.). 



Fig. 145. — The Head of the Water-flea, Daphma 



The compound eye is seen above with several of its 22 omniatidia appear- 

 ing as rounded facets in a bed of pigment. Two of the 4 ocular muscles are 

 also seen encircling the eye. 



Underneath, the pigmented spot is the composite ocellus -^'hich lies in 

 the mid-line; it is made up of the fusion of 3 ocelli (E. F. Fincham) 

 (see Fig. 131). 



The compound eyes of the tiny Branchiopods and some Ostracods are 

 relatively primitive organs with poorly formed ommatidia. The compound 

 eye of the water-flea, Daphnia, may be taken as representative (Fig. 145). It 

 is composed of 22 rudiinentary ommatidia arranged in a sphere of pignient 

 and represents the fusion of two lateral eyes. The eyes of other Branchiopods 

 are often more elaborate, Leptodora, for example, having 300 facets and Poly- 

 phemus 160. In those Ostracods which possess compound eyes, the organs are 

 sometimes separate (paired) if the median composite ocellus is present, but 

 fused if the latter is lacking. On the average they possess between 4 and 50 

 ommatidia (Cypridinse, etc.). 



The compound eyes of Malacostraca consist of ommatidia built 

 upon the standard plan of a cuticular cornea, a crystalline cone, and a 



1 p. 158. 



Leptodora 



Polyphemus 



