CRUSTACEA. 175 



in the large existing Entoraostracan. The eyes are n:ore elevated in 

 the Trilobites. In the Asaphus caudatus the cornea is divided into 

 at least 400 compartments, each supporting a circular prominence : 

 its general form is that of the frustum of a cone incomplete toward^s 

 the middle line of the head, but commanding so much of the horizon 

 in other directions, that where the distinct vision of one eye ceases,, that 

 of the other begins. 



In the mandibulate Crustaceans, distinguished by having their 

 compound eyes supported on moveable peduncles, the form of the 

 corneal facets varies ; they are square in the river craw-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 contin- 

 uous with the inner layer of epiderm, and pierced for the passage of 

 the optic nerve {fig. 90. e). In the Podophthalmous Crustacea there 

 is generally a spacious furrow or cavity, in which the eye and its pe- 

 duncle can be lodged and protected, and it is termed the orbit. In one or 

 two species 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 respecting the state of the world 

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

 sent 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 

 were 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 sufficient at the bottom, 

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



