l6o THE SENSES OF ANIMALS 



connected to an optic nerve. Nevertheless, they do not 

 apparently form images but are merely highly developed 

 light-gatherers, so that the power of vision in the scallop is 

 little better than in the oyster. In a few other species of 

 mollusc the eyes are gathered into bunches so that compound 

 eyes are formed, but here again they appear to be no 

 more than highly organized light-gatherers. (See Plate 



Lastly, in some of the "Coat-of-mail" shells, slug-shaped 

 marine molluscs in which the shell consists of a number of 

 jointed convex plates on the back, we find the greatest pro- 

 fusion of eyes among the molluscs. Although the eyes are 

 minute some of them contain a crystalline lens ; they are 

 lodged in small cavities all over the shell-plates of the back 

 and a nerve from each passes through a minute hole be- 

 neath. In some species they are scattered irregularly, but in 

 others they are arranged in rows in definite patterns ; they 

 may number as many as 12,000 in one animal. 



In one division of the invertebrates we find eyes which, 

 although based on the principle of the lens and retina, have 

 evolved in a very different way to form compound eyes. 

 These animals are the arthropods, the animals in which 

 the hard skeleton is outside and all the muscles, viscera 

 and other soft parts are inside. The most numerous of the 

 arthropods are the insects, but the division includes the 

 Crustacea, spiders, scorpions, millipedes and many others. 

 Not all of these possess compound eyes — many have only 

 simple eyes or ocelli, and yet others have both simple and 

 compound eyes. 



Ocelli are called simple eyes to distinguish them from 

 compound eyes, but their structure is often anything but 

 simple. The lens is formed from thickened layers of the outer 

 cuticle or from large transparent cells derived from the 

 cuticle-forming cells ; a light-sensitive retina consisting of 

 pigment and nerve endings lies behind the lens. There is 

 no chamber filled with semi-fluid jelly as in the vertebrate 

 eye, but a layer of transparent supporting cells separates 

 lens and retina. Ocelli are fixed and immovable, and there 

 seems to be no mechanism for adjusting the focus for near 

 or distant objects. Ocelli of simple structure appear to be 



