CRCSTACK4. 313 



fish, hexagonal in the hermit and common crabs. There is a conical 

 crystalline lens behind each facet imbedded in a small vitreous 

 humour, upon which the optic filament expands, and each ocellus is 

 lodged in a pigmental cell, which likewise covers the bulb of the 

 optic nerve ; the cavity containing the compound eye is closed behind 

 by a membrane continuous with the inner layer of epiderm, and 

 pierced for the passage of the optic nerve {Jig. 131. e). In the 

 podophthalmous Crustacea there is generally a spacious furrow or 

 cavity, in which the eye and its peduncle can be lodged and protected, 

 and it is termed the orbit. In one or two species, e. g. Gelasimus 

 felescopicus, the eye-stalks project beyond the margins of the 

 carapace.* 



One of the most valuable and interesting results of the study of 

 the comparative anatomy of the eye in the Crustacea is the insight 

 which the fossilised remains of similarly constructed organs of vision 

 in the extinct Crustacea have given into the state of the world 

 at the time when they existed ; and I cannot better conclude the 

 present discourse than in the eloquent language of the geologist who 

 first taught the value of the evidence in question. The eyes of the 

 Trilobites of the transition rocks, and those of their nearest congeners, 

 the fossil Limuli from the Carboniferous series, "give information," 

 says Dr. Buckland, "regarding the condition of the ancient sea and 

 ancient atmosphere, and the relations of both these media to light, at 

 the remote period when the earliest marine animals were furnished 

 with instruments of vision in which the minute optical adaptations 

 wei'e the same that impart the perception of light to Crustaceans now 

 living at the bottom of the sea. 



" With respect to the waters wherein the Trilobites maintained 

 their existence throughout the entire period of the transition formation, 

 we conclude that they could not have been that imaginary turbid and 

 compound chaotic fluid, from the precipitates of which some geologists 

 have supposed the materials of the surface of the earth to be derived ; 

 because the structure of the eyes of these animals is such, that any 

 kind of fluid in which they could have been sufiicient at the bottom, 

 must have been pure and transparent enough to allow the passage of 

 light to organs of vision, the nature of which is so fully disclosed by 

 the state oiF perfection in which they are preserved. With regard to 

 the atmosphere, also, we infer that had it differed materially from its 

 actual condition, it might have so far affected the rays of light, that 

 a corresponding diffjrence from the eyes of existing Crustaceans 

 would have been found in the organs on which the impressions of 

 such rays were then received. 



» CCXXXVI., p. 78., pi. xxiv. fig. 1. 



