Page:Geology and Mineralogy considered with reference to Natural Theology, 1837, volume 1.djvu/306

 302 vision, each eye is nearly hemispherical, and placed on a peduncle, by which it is projected to the distance requisite to effect this purpose. (See Pl. 45, Fig. 3, b, and b'.)

In the Serolis (Pl. 45, Fig. 6. b'.), the disposition of the eye, and its range of vision, are similar to those in the Trilobite; but the summit of the eye is less elevated; as the flat back of this animal presents little obstruction to the rays of light from surrounding objects.

In the Limulus (PL 45, Fig. 1.), where the side eyes (b, b') are sessile, and do not command the space immediately before the head, two other simple eyes (b″) are fixed in front, compensating for the want of range in the compound eyes over objects in that directions.

In the above comparison of the eyes of Trilobites, with those of the Limulus, Serolis, and Branchipus, we have placed side by side, examples of the construction of that most delicate and complex organ the eye, selected from each extreme, and from a midway place in the progressive series of animal creations. We find in Trilobites of the Transition rocks, which were among the most ancient forms of animal life, the same modifications of this organ which are at the present time adapted to similar functions in the living Serolis. The same kind of instrument was also employed in those middle periods of geological chronology when the Secondary strata were deposited at the bottom of a warm sea, inhabited by Limuli, in the regions of Europe which now form the elevated plains of central Germany.