The Senses of Animals. 289 



they consist of a lens produced by the thickening of the 

 integumentary layer which is at the same time rendered 



transparent. Behind 

 this lies the so-called 

 vitreous body, com- 

 posed of transparent 

 cells, and then follows 



-Eyes and eyelets of bee. the retina, in which 



A. Drone. B. Worker. there are & numDer f 



rods, the recipient ends of which are turned towards the 

 rays of light, and not away from them as in the vertebrate. 

 Spiders have from six to eight ocelli, arranged in a pattern 

 on the top of the head. Facetted eyes are not found in 

 them. 



These facetted eyes, which are found in both insects 

 and Crustacea, have apparently a more complex structure 

 than the ocelli. Externally — in the bee, for example — the 

 surface is seen to be divided up into a great number of 

 hexagonal areas, each of which is called a facet, and forms 

 (in some insects, but not in all) a little lens. Of these the 

 queen bee has on each side nearly five thousand ; the worker 

 some six thousand ; and the drone upwards of twelve 

 thousand; while a dragon-fly (JEschna) is stated to have 

 twenty thousand. Beneath each facet (in transverse section, 

 Fig. 37) is a crystalline cone, its base applied to the lens, its 

 apex embraced by a group of elongated cells, in the midst 

 of which is a nerve-rod which is stated to be in direct con- 

 nection with the fibres of the optic nerve. Dark pigment is 

 developed around the crystalline cones. And retinal purple 

 is said to be present in the cells which underlie it. 



With regard to these facetted eyes there has been much 

 discussion. The question is — Is each facetted organ an 

 eye, or is it an aggregate of eyes ? To this question the 

 older naturalists answered confidently — An aggregate. A 

 simple experiment seems to warrant this conclusion. If 

 the facetted surface be cleared of its internal structures 

 (the crystalline cones, etc.) and placed under the microscope, 

 each lens may, at a suitable distance of the object-glass, 



u 



