Fig. [138].—Phacops latifrons, Bronn, × 1. Devonian. Showing large compound eye. (After Zittel.)
A pair of compound eyes are present in the majority of Trilobites. Each eye is situated on the free cheek, at that part of its inner margin where the facial suture bends to form an angle (Figs. [137], A, h, 138). The position of the eye is consequently determined by the position of the facial suture; it may be near the glabella or near the lateral margin of the head, and either as far forward as the first segment of the glabella or nearly as far back as the neck-furrow. In many Trilobites the eye is more or less conical, with its summit truncated or rounded, but in some genera it is ovoid, or crescentic. In Aeglina (Fig. [150], H) the eye is flattened and scarcely raised above the general level of the cheek. The eye of a Trilobite is oriented so that its longer axis is parallel or nearly parallel to the axis of the body (Fig. [150], G); but in one case (Encrinurus intercostatus) it is placed at right angles to this axis. The size of the eye varies considerably; it is largest in Aeglina, in which it covers nearly the whole of the free cheek; it is small in Acidaspis and Encrinurus.
Though the eye is always entirely on the free cheek, the adjoining part of the fixed cheek is raised to form a buttress on which the eye rests; this buttress, which is known as the “palpebral lobe,” is seen clearly when the fixed cheek is removed. The eyes of Trilobites are always sessile; for although in some species, such as Asaphus cornigerus, A. kowalewskii, and Encrinurus punctatus, they are on the summits of prominent stalks, yet those stalks are immovable.
Three types of compound eye have been recognised in Trilobites[[184]]—holochroal, prismatic, and schizochroal.
Fig. [139].—Eyes of Trilobites. (After Lindström.) A, B, Sphaerophthalmus alatus, Ang. Upper Cambrian. Vertical and horizontal sections, × 100. C, Asaphus fallax, Dalm. Horizontal section, × 60. D, Nileus armadillo, Dalm. Vertical section, × 60, a, prismatic lenses; b, cuticle; c, part of free cheek. E, Dalmanites vulgaris, Salt. Part of eye, × 30. F, Dalmanites imbricatulus, Ang. Vertical section of eye, with a part of the free cheek on the left, × 60. G, H, Harpes vittatus, Barr. G, The two lenses of one eye, × 8; H, vertical section of the same, × 60.
1. In the holochroal eye (Fig. [139], A, B) the lenses are globular or biconvex and close together, so that the cornea is continuous over the entire eye. Examples of this are seen in Bronteus and Sphaerophthalmus.
2. In the prismatic type (Fig. [139], C, D) the lenses are prismatic and plano-convex, and the entire surface of the eye is covered by a smooth cuticle. The lenses are close together and usually hexagonal, but occasionally rhombic or square. Near the margin of the eye the lenses may become irregular, giving rise to a border in which the prismatic structure is more or less indistinct. The prismatic type of eye is found in the genera Asaphus, Nileus, Illaenus, etc.
3. The schizochroal eye occurs only in the family Phacopidae (Fig. [139], E, F). The lenses are biconvex and are separated by portions of the cephalic shield, so that each lens appears to rest in a separate socket, and the cornea is not continuous for the entire eye. The lenses are circular in outline, but owing to the upward and inward growth of the interstitial test they may appear, on the surface, to be hexagonal. The diameter of a lens may be as much as 0·5 mm. The crystalline cones have not been preserved. In specimens of Phacops rana, in which the inner face of the lens is more convex than the outer, J. M. Clarke[[185]] has obtained evidence of a posterior spheroidal cavity in addition to the anterior corneal cavity. The complete separation of the lenses in this type of eye has led to the suggestion that the schizochroal eye is an aggregate rather than a compound eye. But the difference between this and the holochroal eye is probably less than appears at first sight if the statement made by Clarke is confirmed, namely, that in young specimens of Calymene senaria the lenses are relatively large and similar to those of Phacops, whereas in the adult the eye is holochroal.