The simplest eyes found among the Trilobita are the ocelli. These consist of a Simple thickening of the test to form a convex surface capable of concentrating light. The similarity in position of the paired ocelli of trilobites and the simple eyes of copepods has perhaps a significance.
The schizochroal eyes may well be compared with the aggregate eyes of the chilopods and scorpions. The mere presence of a common external covering is not sufficient to prove this a true compound eye, especially as the covering is merely a continuation of the general test.
The holochroal eyes are of two kinds, one with plano-convex and one with biconvex lenses. The latter would seem to be mechanically the more perfect of the two, and it is worthy of note that the trilobites possessing the biconvex lenses have, in general, much smaller eyes than those with the other type.
If, as some investigators claim, the parietal eye of Crustacea originates by the fusion of two lateral ocelli, trilobites show a primitive condition in lacking this eye, which may have originated through the migration toward the median line of ocelli like those of the Trinucleidæ.
That the sexes were separate in the Trilobita there can be very little doubt, but the study of the appendages has as yet revealed nothing in the way of sexual differences. One of the most important points still to be determined is the location of the genital openings.
In many modern Crustacea, the antennæ or antennules are modified as claspers, and it is barely possible that the curious double curvature of the antennules of Triarthrus indicates a function of this sort. The antennules of many specimens have the rather formal double curvature, turning inward at the outer ends, and retain this position of the frontal appendages, no matter what may be the condition of those on the body. Other specimens have the antennules variously displaced, indicating that they are quite flexible. It is conceivable that the individuals with rigid antennules are males, the others females.
It is interesting to note that the antennules of Ptychoparia permulta Walcott (1918, pl. 21, fig. 1) have the same recurved form. All the specimens of Neolenus, however, show very flexible antennas.