672 GEOLOGY. 



literally teem with them. Though long supposed to be insects, the trilobites are now 

 known to be true crustaceans ; and, though the entire family is without any living re 

 presentative, having been annihilated with the accumulation of the carboniferous deposits, 

 we can make as near an approximation to their habits as we can to those of many of tlio 

 inhabitants of the existing seas. The existing crustacean which most resembles the 

 extinct trilobite appears to be the Bopyrus, a small parasitical animal attaching itself to 

 prawns, and causing a large swelling on their bodies, one parasite fastening itself on each 

 prawn. 



The trilobites are usually found from one to six inches long ; but a specimen discovered 

 in the state of Ohio, and described by Professor Locke, was nearly two feet in length. 

 These animals had a protecting crustaceous shell, or case, composed of numerous segments, 

 folding over one another like those of a lobster's tail. This shield they appear to have had 

 the power of contracting at pleasure, like the wood-louse and armadillo, as some are found 

 variously expanded or coiled up. No certain traces of antennae and legs have been dis 

 covered ; from which a sedentary habit is inferred, the animals adhering, with a soft 

 articulated underside, either to rocks and fuci, or in masses to one another, hence forming 

 those conglomerations of individuals which are so remarkable in various rocks. Some 

 naturalists, however, suppose a power of locomotion in the water, either by soft paddles, 

 which have perished, or by sculling forward with the aid of a flexible extremity. 



From a peculiarity of the mouth, it has been concluded that they were carnivorous, 

 preying on naked molluscs, or on the annelides, with which their remains are asso 

 ciated. 



The most curious fact respecting the trilobites is the compound organisation of the eye 

 in several species, many specimens of which have been found in a perfect state of pre 

 servation, and unfold a structure of which we have various living examples. It is well 

 known that the eyes of crustaceous animals, like those of insects, are composed of a vast 

 number of minute facets or lenses, placed at the end of tubes, which are arranged side by 

 side, so as to produce a radiating mass of eyes, which being generally of a hemispherical 

 or conical form, and sometimes elevated from the head on a stem, enable the animal to 

 see in every direction. In some insects the number of these lenses in both eyes, as in the 



house-fly, is 14,000 ; in other cases, 

 the dragon-fly, 25,000 ; in others, the 

 butterfly, 35,000 ; in others, the Mor- 

 della, 50,000. The visual organ of 

 several of the trilobites is formed 

 upon the same principle, each eye, ac 

 cording to fossil anatomists, contain- 



Eyeofthe Trilobite. ing at least 400 spherical lenses, fixed 



in separate compartments on the surface of a cornea projecting conically upwards. The 

 engraving is a representation of this structure. 



The preceding facts lead us to results of singular interest and importance. They 

 establish the conclusion, that at that remote epoch when the trilobites flourished, countless 

 ages ago, the condition of the atmosphere and of the waters of the sea, with the adaptation 

 of light to the eye, and of the eye to light, were much the same as at the present era. 

 " With respect to the waters," says Dr. Buckland, in his luminous commentary upon the 

 phenomena of these crustaceans, " we conclude that they 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 of 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 difference from the 



