THE LOBSTER AS A TYPE OF CRUSTACEA 21 



divided up into a beautifully regular series of square 

 facets. This membrane is a thin and transparent 

 continuation of the chitinous covering of the body, 

 and if it be stripped off and examined under a micro- 

 scope, it will be found that each facet is capable of 

 acting as a lens and forming an image of external 

 objects. It is not to be supposed, however, that the 

 Lobster sees a separate image in each of the facets, 

 some thirteen thousand in number, which go to 

 make up each eye. In the interior of the eye, at 

 some distance from the surface, are a large number 

 of rod-like bodies, connected with the fibres of the 

 optic nerve, and believed to be the actual organs for 

 the perception of light. Each rod corresponds to 

 one of the facets, and as it lies at the bottom of a 

 long conical tube, of which the walls are covered 

 with dark pigment, it can only receive light from a 

 single point in line with the axis of the tube. In 

 this way the image of any object will be built up, 

 like a mosaic, out of the impressions of light and 

 darkness received through the separate facets, and 

 transmitted to the underlying rods. It has been 

 shown in some Crustacea that, when the animal is 

 in a very dim light, the curtain of pigment separating 

 the tubes is partially withdrawn, so that the light 

 from each facet can reach, not one, but several rods. 

 In this way the images of objects received are much 

 brighter, although they are less sharply defined. 

 It might be thought that in animals like the 



