152 



THE EYE IN EVOLUTION 



COMPOSITE OCELLI 



COMPOSITE OCELLI (SIMPLE EYEs) are formed by the fusion of two 

 or more ocelli each with its own retina and pigment cnp, a process 

 which seems to have arisen independently in several phyla ; in 



Figs. 130 to 132. — The Composite Ocellus. 



Cypris 



Fig. 130. — The ocellus of Cypris. 



Fig. 131. — The ocellus of Daphnia. 



The unpaired median eye represents the fusion of 3 ocelli (see Fig. 228). DL, 

 dorso-lateral ocelli; VE, ventral ocellus; P, pigment mantle; V, visual cells; 

 T, tapetum (after Claus, 1891). 



EDIAL 



VENTRAL 



Fig. 132. — The ocellus of the chsetognath, Spadella exaptera. 

 Showing 3 of the 5 simple eyes, one to the left and 2 to the right, arranged 

 round the central pigment, P. Ep, epithelium ; V, visual cells ; R, rods ; 

 N, nerve fibres (after Hesse). 



general, the fusion is associated with degeneracy and lack of use. It is 

 interesting that the same cyclopic tendency is seen in the median 

 (pineal) eye of Vertebrates, which initially was a paired organ. ^ Among 

 certain smaller Crustaceans, lowly types which have undergone much 

 reduction of the head and have largely lost their segmentation, a 

 median unpaired eye is a characteristic feature, and is frequently 

 composed of the fusion of a number of ocelli arranged in a somewhat 

 similar way (the Cladoceran, Daphnia ; the Ostracods, Cypris and 

 Cypridina ; the Copepod, Cyclops) (Figs. 130 and 131). ^ Among the 

 marine arrow-worms (Chsetognatha), Spadella has two composite ocelli 

 near the anterior extremity of its body, each organ made up of the 

 fusioi! >f 5 simple eyes of the cupulate type arranged around a central 



Cyclops 



1 p. 711. 



p. 163, Fig. 145. 



