[488] Ibid. p. 667.
The habits of the Trilobites, as deducible from Mr. Macleay’s exposition of their structure and affinities, must have resembled those of the Cymothoadæ, some of which, like the Calymenes, coil themselves up, and are not parasitical; while their close affinity to Bopyrus, and the apparent absence of distinct crustaceous feet, imply that they were to a certain degree sedentary. The flat under surface of their bodies, and the lateral coriaceous margin of several species, which is so analogous to that of the multivalve shell Chiton, render it probable that they adhered by a soft, articulated, under surface, to the rocks or sea-weeds. Their instruments of progression are unknown; whether they moved by means of membranaceous feet, or by the undulations of setigerous segments, like the earth-worm, or by wrinkling the under surface of the abdomen, like the Chiton, are questions yet to be determined. It is evident, from their longitudinally trilobed form, and lateral coriaceous margin, that they had the power of firmly adhering to flat surfaces; and while thus sedentary the thin but hard dorsal crustaceous shell would protect them from the attacks of their enemies. "The Trilobites, probably, like the Chitones, adhered in masses one upon another, and thus formed those conglomerations of individuals which are so remarkable in certain rocks; but it is not likely that they were parasitical, since almost all the existing parasites that adhere to other animals, have strong feet, armed at their extremities with hooks for that purpose."[489] From the form of the labrum of the mouth (Barrande, pl. i. and ii A) it is inferred that they were carnivorous, preying on naked mollusks, or on the annelides, with which their remains are associated.
[489] Murch. Sil. Syst. p. 669.
As the compound eyes of the Trilobites[490] are similar to those of existing crustaceans and insects (see Wond. p. 792), the highly interesting and important fact is established, that the mutual relations of light to the eye, and of the eye to light, were the same in the remote epoch when the Trilobites flourished, as at the present time; and that the condition of the waters of the sea, and the atmosphere, and the relation of both these media to light, have undergone no change through the countless ages that have elapsed since the deposition of the Silurian strata.[491]
[490] The compound eyes in many specimens remain in a high state of preservation. M. Barrande in the eye of a Brontes palifer counted 30,000 lenses. See also Barrande, pl. iii.
[491] See Dr. Buckland’s eloquent and instructive commentary on this subject, Bd. pp. 401-404.
Geological Distribution of Fossil Crustaceans. We have seen that the Tertiary strata contain the remains of many of the highest organized crustaceans; a few brachyurous, macrurous, and entomostracous genera appear in the Cretaceous, Oolitic, and Liassic formations; whilst the Isopodous Archæoniscus and several species of Cypris occur in the Wealden and Purbeck deposits. Some few Entomostraca have been enumerated from the Trias and Permian.
One species of macrurous decapod has been found in the Muschelkalk of Germany; and Mr. Prestwich’s "Apus dubius" (Geol. Trans. 2d ser. vol. v. pl. xli. fig. 9), and Dr. Ick’s crustacean, noticed in Journ. Geol. Soc. vol. i. p. 199, both from the Coal Measures of England, are probably true Decapodous Crustaceans. With these exceptions not a species of the numerous tribes of Crabs, Lobsters, &c. has been observed in the older formations, though composed of such enormous thicknesses of marine detritus, and containing countless myriads of the relics of the inhabitants of the ocean.