ARTHROPODA 



209 



sometimes associated with a single compound eye, as in Daphnia, 

 sometimes with paired lateral eyes which may be either simple, as in 

 PonteUopsis, or comp>ound in type, as in the Phyllopod, Apus (Fig. 204). 

 In Apus the median eye is really a paired organ but the two are so 

 closely situated that they appear on examination to be a single spot. 

 The median e^-e of these small Crustaceans is situated either dorsal 

 or ventral to the cervical ganglion and is of the composite simple 

 type ^ ; it is comjDosed of the fusion of a number of constituent 

 ocelli (usually 3). Such a median eye is present in most of the 

 Branchiopods and Ostracods, only occasionally degenerating when 

 the compound eyes are particularly well developed {Polyphemus, 

 Leptodora). 



The ocular arrangements in these actively swimming small Crustaceans is 

 therefore very varied. The eyes of the water-flea, Daphnia, may be taken as 

 representative of the Branchiopods and Ostracods. There is a single compound 

 eye in the mid-line composed of 22 relatively rudimentary ommatidia (Fig. 206). 

 Behind and below this, buried in the central nervous system, is the small 

 composite ocellus (Figs. 131 and 145). It is interesting that the compound 

 eye is actively motile, being kept in a state of continual vibration by 4 muscles 

 somewhat resembling in their arrangement the rectus muscles of vertebrates 

 (Rabl, 1901 ; Hess, 1912). It would seem that the small composite ocellus is 

 of little functional value. The phototactic responses exhibited by the animal 

 depend entirely upon the more elaborate compound eye ; when this has been 

 removed the phototactic responses fail although the more primitive generalized 

 sensitivity to light persists (Schulz, 1928 ; Harris and ^lason, 1956). 



The eyes of some of the actively swimming Copepods take on another form. 

 In the female PonteUopsis regalis, there are two very small dorsal ocelli sym- 

 metrically placed and a large unpaired median eye situated fronto -vent rally 

 underneath the rostrum ; it has a large cuticular lens and 6 retinal cells arranged 

 in an inverted position in two groups of 3, forming an intermediate step between 

 a simple eye and an ommatidiura (Vaissiere, 1954-55). The elongated, actively 

 motile eyes of Copilia are of the same general structure with a retinule of 3 

 sensory cells (Fig. 139) (Grenacher, 1880-95 ; Exner, 1891). This animal has two 

 such eyes facing forwards and widely separated ; in Sapphirina they are 

 close together ; and in Corycceus so close that the lenses ai-e fused in the 

 mid-line. 



Polyphemus 



Copilia 



Balanus 



In sessile forms eyes are usually present in the actively swimming 

 nauplius stage ; thus in the acorn-shell, Balanus. there is initially a 

 median unpaired eye but after several moults in the pupal stage two 

 lateral composite eyes are acquired. In adult life, however, these 

 become vestigial, as also does the unpaired eye of the ship-barnacle, 

 Lepas (Fales, 1928). In some parasitic forms such as the fish-louse, 

 Caligus, both median (sim^^le) and lateral (composite) eyes are also 

 present, but in degenerate types such as SaccuJina eyes and other 

 sense organs are lost. 



1 p. 152. 



Lepas 



S.O. —VOL. I. 



