chap, xii.] OPTIC ORGANS. 457 



Movements of the eyes. It is clear that when 

 the number of eyes becomes limited, the power of 

 sight of their possessor must either be very small, 

 or the eye must acquire a power of movement. To 

 obviate this inconvenience various means have been 

 resorted to. The eye may, as in polymeniscous 

 Arthropods, be provided with a large number of 

 lenses, so that the whole corneal surface extends 

 over more than half a sphere ; in addition to this, 

 the body may be provided with less useful lateral 

 eyes, as in the scorpion ; or, as in the crayfish or 

 the crab, the eye may be placed at the end of a 

 movable stalk. Phenomena of a corresponding kind 

 obtain among the Mollusca ; the Lamellibranchiata, 

 which are without pra?stomial eyes, often have a 

 number of small eyes (or pigment spots only) deve- 

 loped on the edges of the mantle, and these even are 

 sometimes placed on stalks. In some Gastropods the 

 eyes are placed at the ends of the tentacles, and as 

 these tentacles are capable of protrusion and retrac- 

 tion, the optic nerve is of sufficient length to be quite 

 straight only when the tentacle is protruded, while, 

 when that organ is retracted, the nerve is looped. 

 In Onchidiuni and some of its allies, a number of 

 simple eyes, resembling in essential arrangement those 

 of the Yertebrata, are developed on the surface of the 

 back of these shell-less and slow-moving molluscs ; 

 Semper has counted as many as ninety-eight of these 

 eyes 011 the back of an Onchidium. 



In the Chitonidas, Moseley has recently detected 

 in some species more than ten thousand minute eyes, 

 placed on the exposed surfaces of their shells ; but it 

 is remarkable that these eyes, unlike those of Onchi- 

 dium, are on the type of the invertebrate, and not of 

 the vertebrate, eye. Scattered among them are tactile 

 organs, from which, it is supposed, the eyes have 

 arisen by modification. In some of the heteropodous 



