SIGHT 



45 



Dasychone) there are eyes of elaborate nature on the gill-filaments 

 of the head. 



Spiders, among Arthropods, have a group of simple eyes 

 (ocelli) on the top of the head. These are constructed on the 

 camera principle, though they differ in detail from those of Alciope 

 (fig. 1058). The spherical shape of the lens and its closeness to the 

 retina suggest that only near objects can be seen with any degree 

 of distinctness. A great many Insects possess ocelli in addition 

 to the two large compound eyes. In Bees, for instance, there are 

 three of these arranged in a triangle on the top of the head. In 

 this and similar cases it is extremely probable that the compound 



nf. 



Fig. 1059. Diagrammatic Sections through Camera Eyes of Cephalopods 



A, Eye of Nautilus; c., internal cavity; r. and ^., refracting and sensitive layers of retina; n.c., layer of nerve-cells; 

 n.f., nerve-fibres. B-D, Stages in development of eye of Cuttle- Fish (Sepia); in B the epidermis (ep.) has folded in to 

 produce a vesicle (v., v.); in c a fold (if.) is growing out to form the iris; D is the adult eye; /., protective external 

 fold; *'., iris; /., /., outer and inner parts of lens; r. and s., refracting and sensitive layers of retina. 



eyes are used for seeing things at a distance, while the ocelli are 

 used at close range. As focussing arrangements are entirely absent 

 this would certainly be a great convenience. 



The most familiar example of camera eyes among Molluscs 

 is afforded by the Garden- Snail (Helix aspersa, fig. 1038), where 

 they are placed near the tips of the long front tentacles. It is 

 extremely short-sighted, as we might expect, in view of the fact 

 that the lens is practically spherical and very close to the retina. 

 The Pearly Nautilus possesses eyes which are constructed on the 

 "pinhole camera" principle. There is no lens, and sea- water is 

 admitted by a minute hole into the large internal cavity (fig. 1059). 

 Large and complex eyes are found in the rapacious Squids and 

 Cuttle- Fishes, and some idea of their structure and mode of de- 

 velopment will be gathered from fig. 1059. A few of the Bivalve 

 Molluscs possess numerous complex camera eyes situated on the 

 edges of the mantle-flaps, as in the Scallops (Pecten), where they 



