There has been a rather general impression among students of trilobites that the eye-lines, which should be differentiated from the genal cæca, denote the course of the optic nerves, but no other evidence of the nervous system has been found, save the so-called nervures which have been discussed above. In _Apus_ the nerves leading to the eyes come off from the anterior ganglion or "brain" and run directly to the eyes. If conditions were similar in the trilobites, the "brain" was beneath the anterior glabellar lobe, provided, of course, that the eye-lines do indicate the course of the optic nerve. The ontogenetic history of the eye-lines of trilobites with compound eyes is instructive, and has already been discussed by Lindstroem (1901, pp. 12-25), but he did not cite the case of _Ptychoparia_, which is particularly interesting, because in this genus both eye-lines and "nervures" are present. Beecher (1895 C, p. 171, pl. 8, figs. 5-7) has shown that in _Ptychoparia kingi_ the eye-lines of a specimen in the metaprotaspis stage run forward at a low angle with the glabella, while in the adult their course is nearly at right angles to it. They have therefore swung through an arc of at least 60 and in so doing have had ample opportunity to become coincident with the primary trunks of the genal cæca. Once that was accomplished, it is quite likely that the one fold in the shell would continue to house both structures. In other trilobites, there is a similar backward progression of the eye-lines. As would be expected, the ventral ganglia and the longitudinal cords left no trace in the test. Since each segment has appendages, there was probably a continuous chain of ganglia back to the posterior end of the pygidium. VARIOUS GLANDS. _Dermal glands._--The surface of many trilobites is "ornamented" with pustules and spines which on sectioning are nearly always found to be hollow, and in many cases have a fine opening at the tip. While it is generally believed that the purpose of these spines was protective, yet it is possible that many of them were merely outgrowths which increased the area through which the respiratory function could be carried on. It will be recalled that most of the smooth trilobites are punctate, some of them very conspicuously so, and the spines and pustules of ornamented trilobites may merely subserve the same function as the pores of smooth ones. If the spines were protective, it would not be surprising if some of them, hollow and open at the top, were poisonous also, and had glands at the base. These are, however, purely matters of speculation so far. _Renal excretory organs._--Nothing has been seen of any such organs, unless the genal cæca may possibly be of that nature. The main trunks of these always lead to the sides of the anterior glabellar lobe, which is not the point of attachment of either antennæ or biramous limbs, so that there seems little chance that they will bear this interpretation.