§ 22. VISUAL ORGANS OF THE TRILOBITES. 793 



up of a series of optical tubes, or telescopes, the number of 

 which in some insects is quite marvellous. Thus, each eye 

 of the common house-fly is composed of eight thousand 

 distinct visual tubes ; that of the dragon-fly, of nearly 

 thirteen thousand ; and of a butterfly, of seventeen thousand. 

 That any traces should remain of the visual organs of 

 animals which existed at so remote a period seems at first 

 incredible ; but there are no limits to the wonders which 

 Geology unfolds to us.* The Trilobite, like the Limulus, 

 was furnished with two compound eyes, each being the 

 frustrum of a cone, but incomplete on that side which is 

 opposite to the other. In the Asaphus (Lign. 179), four 

 hundred spherical lenses have been detected in each eye ; 

 but in general the lenses have fallen out, as often happens 

 after death in the eyes of the common lobster. " Thus," 

 observes Dr. Buckland, " we find in the trilobites of these 

 early rocks, the same modifications of the organ of sight as 

 in the living crustaceans. The same kind of instrument 

 was also employed in the intermediate periods of our 

 geological history, when the secondary strata were de- 

 posited at the bottom of a sea inhabited by Limuli {ante, 

 p. 738), in those regions of Europe which now form the 

 elevated plains of central Germany. But these results are 

 not confined to physiology : they prove also the ancient 

 condition of the seas and atmosphere, and the relation of 

 both these media to light. For in those remote epochs, the 

 marine animals were furnished with instruments of vision 

 in which the minute optical adaptations were the same as 



* The structure of the eye of the Trilobite was, I believe, first 

 noticed by that accurate observer, Mr. Martin, the author of Petri/. 

 Derbiensia. In the work of my friend, M. Brongniart (Histoire 

 Naturelle des Crustaces Fossiles, par A . Brongniart etG.A. Desmarest, 

 1 vol. 4to. with Eleven Plates, Paris, 1822), the eye of the Trilobite is 

 beautifully represented. In Dr. Buckland's Bridgewater Essay, the 

 subject is ably elucidated, and placed before the reader in a striking 

 point of view. 



