DISPOSITION OF THE LENSES. 301 



the disposition of the facets, obvious examples of optical 

 adaptation. 



In the Asaphus caudatus (see PI. 45, Figs. 9 and 10.,) 

 each eye contains at least 400 nearly spherical lenses fixed 

 in separate compartments on the surface of the cornea.* 

 The form of the general cornea is peculiarly adapted to the 

 uses of an animal destined to live at the bottom of the water: 

 to look downwards was as much impossible as it was unne- 

 cessary to a creature Jiving at the bottom ; but for horizon- 

 tal vision in every direction the contrivance is complete.f 

 The form of each eye is nearly that of the frustum of a cone 

 (see PI. 45, Figs. 9 and 10.,) incomplete on that side only 

 which is directly opposite to the corresponding side of the 

 other eye, and in which if facets were present, their chief 

 range would be towards each other across the head, where 

 no vision was required. The exterior of each eye, like a 

 circular bastion, ranges nearly round three-fourths of a cir- 

 cle, each commanding so much of the horizon, that where 

 the distinct vision of one eye ceases, that of the other eye 

 begins, so that in the horizontal direction the combined range 

 of both eyes was panoramic. 



If we compare this disposition of the eyes with that in the 

 three cognate Crustaceans, by which we have been illus- 

 trating the general structure of the Trilobites, we shall find 

 the same mechanism pervading them all, modified by pecu- 

 liar adaptations to the state and habits of each ; thus in the 

 Branchipus (PI. 45, Fig. 3, b, b',) which moves with rapidi- 

 ty in all directions through the water, and requires universal 



* As tlie Crystalline lens in the eyes of Fishes is spherical, and those 

 in the Eye of Trilobites are nearly so, there seems to be in this form an 

 adaptation to the medium of Water, which would lead iis to expect to 

 find a similar form of lens in the compound Eyes of all marine Crustacea, 

 and probably a different form in the compound Eyes of Insects that live in 

 Air. 



f The facetted eyes of Bees are disposed most favourably for horizontal 

 vision, and for looking downwards. — Lib. Ent. Knowl. v. xu. p. 130. 

 VOL. I, — 26 



