The position of the hypostomic eyes on the ventral surface of the trilobites is not quite so abnormous, nor so isolated a feature amongst the crustaceans as might at first be supposed. Amongst the recent crustacea there is probably none, which as to the conformation of its labrum or hypostoma resembles the trilobites so closely as the species of the genus Apus, however different they may else be in all other respects. We have studied chiefly the hypostoma of Apus cancriformis. In its shape as seen from the outside and as to its outlines it reminds strongly of the trilobite hypostoma. It is nearly square ([pl. VI figs. 46, 47]) with rounded corners. The anterior margin is arched and in the middle it has a broad tonguelike prolongation. On the sides of the anterior margin there are two short, sharply pointed wings. The lateral margins are thick, almost as doubled, and bent forward so that a groove is formed alongside them and the large, evenly vaulted median surface of the hypostoma. These grooves are continued in a short groove parallel with the posterior margin. From this margin a narrow oblique surface is slanting backwards, as it were, doubling that margin. On its surface there are two small, resplendent, white or clear spots, almost of the shape of a crescent, though more irregular when seen in higher enlargement ([pl. VI fig. 49]). They are translucent as to be visible on the interior side of the hypostoma. In Apus glacialis there are also similar spots in the same position, but they form short narrow, straight stripes, parallel with the border of the posterior margin. I have no opinion as to the nature or function of these white spots. I do not think that they at all are homologous with the macula; of the trilobites. On the other hand we see a little higher up on the exterior side of the hypostoma two faintly elevated elliptic tubercles which as to their position and shape highly remind of the maculæ of the trilobites. They are hollow on the inside and we have not been able to find any peculiar structure in them. That they have any visual function is precluded through the circumstance that the whole exterior surface is densely covered with microscopic bristles ([pl. VI fig. 49]) which are especially remarkable along the posterior margin.
In the class of the Cirrhipedia a sufficient number of facts is known to ascertain us of the presence of eye spots on the hypostomas of the larva. The researches of Spence Bate[43] show in the larva of Balanus balanoides, on the hypostoma or labrum, which he calls proboscis, an eye, a little from the posterior margin of the labrum. It seems during the various moults of the animal to have changed its place, being in the two first moults situated near the anterior margin of the hypostoma. In Clitia Strömi it lies at the anterior part of the labrum ([figs. 9, 10]). Spence Bate does not accept the opinion of their nature as eyes, but the following authors as Darwin and Claparède agree in considering these spots as eyes. Darwin[44] describes two such eyes in Lepas australis and, if I catch his meaning rightly, one on each side of the labrum. Claparède has given a figure of the larva of Lepas anatifera[45] in the hypostoma of which (by him called »Rüssell», proboscis) a single eye is seen, having at the sides two round problematic organs, that he at first considered as auditory cells, but later as eyes. Hesse[46] has also in a species of Scalpellum seen several coloured spots on the inferior surface of the carapace, below the antennæ and he expressly says that they are eyes and a special pair of supplemental eyes that are becoming functional after the first moult.
[43] On the development of the Cirripedia, Ann. Mag. Nat. list. 1851, p. 324, pl. VI figs. 1, 2, 3, pl. VII fig. 10.
[44] Monogr. of Cirripedia I p. 17.
[45] Beobachtungen ... über wirbellose Thiere, 1863, pl. XVII fig. 22.
[46] Mémoire sur les métamorphoses que subissent les Anatifes appelés Scalpels obliques. Annales des Sciences nat. 1859. Tome XI p. 163.
These sternal eyes in the larva are previous to the eyes in the dorsal side of the later stages and whenever such are discovered in other groups of adult animals as the trilobites they must of course be considered as remnants from the larval stage. As a rule, in several tribes of the crustacea it has been observed that the eyes originate on the ventral or sternal side of the body and in the advancement of growth move to the superior side. So according to Packard and others the embryo of Limulus has originally the median eyes on the ventral side. In the same way the cephalic eyes of Dalmanites move, as Barrande has shown, from the ventral side to their permanent place on the superior side of the head shield. But while in the Cirrhipedia the larval or hypostomic eyes are lost in the adult they remain on the hypostoma of a great number of the adult trilobites and have probably left their mark behind in A pus. No wonder then that we find these maculæ developed as precursors of the hypostomic eyes in a number of the oldest and in other respects blind trilobites as the Olenellidæ and Paradoxidæ to which they may have been of some use as a sort of visual organs, though probably very imperfect.
We find the maculæ of the trilobites present from the oldest Cambrian times and we find also in them a progressive evolution, in some to a high degree, lenses and facets, perfectly identical with those of the eyes on the head shield, converting them into true eyes. It was in the Lower Silurian times, in the Lower grey Orthoceratite limestone of Sweden, that Illænus Esmarki appeared provided with actual prismatic lenses on its maculæ and probably also Lichas. Still higher at the top of the Lower Silurians, in the Leptæna limestone of Dalecarlia the genus Bronteus has appeared with those highly developed hypostomic eyes.
But there are, no doubt, still more facts to adduce for filling up extant lacunæ in the knowledge of these matters. The material now at hand has, however, not been sufficient for doing this. At present I must leave this interesting field of investigation and be contented if I have succeeded in showing where rich harvests await every one who will make a search for them.