COMPOUND EYES IN MOLLUSCS. 141 



through which the water has free access to the interior 

 of the eye. 



In the higher cuttle-fishes (Cephalopoda) the eye is 

 very complex, and the optic ganglion is in some cases 

 the largest part of the brain ; but, while we find the 

 same parts, as, for instance, in Helix, though in a higher 

 state of development, there does not seem sufficient 

 reason to regard the two organs as homologous, but 

 it appears possible that the eye of the cuttle-fish had 

 an independent origin. 



Certain bivalves (Lamellibranchiata) possess bright 

 spots round the edge of the mantle, or on the siphon, 

 which some naturalists maintain to be eyes, while 

 others deny them this character, leaving their true 

 function, however, undecided. 



But though there is much doubt in some cases, there 

 are other eye-spots which are certainly true eyes. Of 

 these there are two distinct types — those of Spondylus, 

 Pecten, etc., on the one hand ; of Area, Pectunculus, etc., 

 on the other. The latter present several features of the 

 compound insect's eye. This was first noticed by Will,* 

 and they have since been more fully described by 

 Carrierej and Patten.if They are composed (Fig. 96) 

 of large conical cells wdth the points turned inwards. 

 Pigment is deposited in the periphery of the cells. 

 The outer surface is arched, and forms a biconvex lens. 

 These cells pass gradually into those of the ordinary 

 epithelium. 



It will be most convenient to consider the mode in 

 which these compound eyes act when we come to 



* " Ueber die Augen der Bivalven," Frorieps Notizen, 1844. 



t "Die Sehorgane der Thiere," 1885. 



X " Eyes of Molluscs and Arthropods," Miit. Zool. Stat. Neapel, 1886. 



