Part 5

As a transition to the next phase forms may be imagined having head and the beginning pygidium, nearly as Agnostus atavus, minus the thorax. The corresponding larval stages are Liostracus (BRÖGGER'S figures IV, V, VI) and Sao (BARRANDE, pl. 7 fig. 2-4 a, b). Nearly so, though the pygidium is more developed, are the larva of Agn. bibullatus and Agn. nudus (BARR., pl. 49) both without thorax.

II.

These have the three chief parts of the body developed.

3. The facial ridge. Cambrian, adult with glabella and facial ridges, short or long, emanating from the top of the glabella, thorax and pygidium. Several American Conocoryphæ. Corresponding larval stage, Sao, stage 3, BARR. pl. 7 figs 4 d-9.

4. Suture. A suture dividing the fixed cheeks in two pair viz. two fixed cheeks and two free cheeks. Fully developed facial ridge. Oldest known adult forms are Ellipsocephalus, Arionellus, already in the Lowest Cambrian, the Olenellus beds, what presupposes a long, antecedent lineage far back in the precambrian times. Corresponding larval stage in Sao, BARR. pl. 7 figs 10-13. The plurality of Cambrian trilobites belong here. An intermediate stage leading to the next is seen in such forms in which the ridge posteriorly is widened into the »eye lobe», which rests in the scallop of the free cheek. So it is in Liostracus and many others besides. Solenopleura possibly oculate.

III.

5. Globular eyes. Cambrian, in the youngest zone, the Olenus schists. The oldest at present known oculate trilobite Eurycare is found in the second division of these schists.

In the lowest Lower Silurian division, the Ceratopyge limestone, Euloma and Ceratopyge occur as the last survivors of the blind, partially ridge bearing genera. The Trinucleidæ and Ampyx belong to another group of trilobites.

Even among other exclusively Lower Silurian genera, in which the plurality of the species is oculate, there are species entirely blind. So with Illænus, in which genus Dr HOLM has not found any eye in Ill. Angelini, I. leptopleura and Ill. cæcus. The free cheek in these three species is much narrow, as the facial suture lies near the margin of the head.

=The eyes of the Trilobites.=

If, as is probable, to judge by the conformity of their cornea with that of recent crustacea, the trilobites like these were provided with crystalline cones beneath the corneal lenses or facets, only the latter have been preserved in a fossil state. Although the crystalline cones in consequence of their solid consistence might have been petrified as well as the cornea, they must, imbedded as these tiny cones lie, entirely wrapped up in delicate tissues, fall away and be lost, when the dissolution of the dead body had set in. Consequently the curious appendages on the inferior side of the lenses in Dalmanites vulgaris (Pl. III f. 50) or Phacops quadrilineata (Pl. V fig. 38) can noways be considered as belonging to the original structure of the eye, apart from their great dissimilarity with anything appertaining to the eyes of the Arthropoda. The cornea on the contrary cohered with the integument of the body, and it has been well preserved in a great number of trilobites.

I subdivide the trilobites in respect to the form of their eyes in the following manner:

=I. Genera with compound eyes.=

1. With prismatic plano-convex cornea facets.

Acidaspis, as a transitional form to the next group. Asaphus. Bumastus. Cyphaspis. Dysplanus. Encrinurus. Illænus. Megalaspis. Nileus. Phillipsia. Niobe. Proetus. Ptychopyge. Symphysurus.

2. With round or biconvex transversally elongate lenses.

Acerocare. Bronteus. Chirurus. Ctenopyge. Cyrtometopus. Eurycare. Peltura. Sphærophthalmus.

=II. Genera with aggregate eyes of biconvex lenses.=

Acaste. Chasmops. Dalmanites. Phacops.

=III. Genera with isolated eyes, one or several stemmata at the extremity of a straight facial ridge.=

Harpes. Harpides. (?) Trinucleus in the larval state.

Only a few authors have before now occupied themselves with the intimate structure of the trilobite eye. PACKARD gave in 1880, in the »American Naturalist» a note on the structure of the eye of trilobites (p. 503). There are some rough and inexact sketches of the eyes of Limulus and Asaphus, and although he seems to have known the beautiful researches of GRENACHER he still »claims that the trilobite eye was organized on the same plan as Limulus». This statement is altogether wrong, and as I hope to show the trilobites have had eyes entirely different from that of Limulus and instead agreeing with those of the Isopoda and perhaps also with a few other Crustacea. In 1889 J. M. CLARKE published an account on the »Structure and development of the visual area in the trilobite Phacops rana GREEN». The aggregate eye described by him are of the type forming my third group. His holochroal division embraces my first and second groups and the schizochroal my third.

The latest contribution to the knowledge of these eyes is found in EXNER'S »Physiologie der facettirten Augen von Krebsen and Insecten», 1891, where he gives good figures of the lenses of Phacops fecundus, pl. II figs 18, 19. He says that the palpable difference in the structure of these eyes and those of Limulus point to a change in the function of these eyes.

I. =Compound eyes.=

=1. Eyes with prismatic, plano-convex lenses.=

A pellucid, smooth and glossy integument, a direct continuation of the common test of the body covers the corneal lenses, quite as is the case in so many of the recent crustacea. In the plurality it is, however, difficult to discern the lenses from the outside.

The lenses, as seen in a vertical section of the eye of Asaphus expansus, (pl. I fig. 12), are columnar prisms, like the pillars of basalt, attaining a length of 0,2 mm and at the point where the eye joins the test of 0,3 mm. At their interior extremity they have a breadth of 0,066 mm. On that point the surface is convex and at the exterior surface plane. They are closely packed and in a transverse section resemble a pavement of regular hexaeders. But they also assume other shapes and become rhombs or even quadrates, as seen in a specimen of Asaphus fallax (pl. I fig. 18), where the hexaeders and quadrates lie side by side without transitional forms. As a rule the lenses become more and more irregular in the vicinity of the surrounding frame or near the suture, nearly blotted out, as it were, and without any definite border line mingled with the confused, spongy mass that like a belt or a frame surrounds they eye in Asaphus and is sharply limited from the other part of the free cheek. This remarkable zone which is almost only present amongst the Asaphidæ (Asaphus, Megalaspis, Ptychopyge, Isotelus) retains in a confused manner somewhat of the prismatic structure of the eye as shown in the section (Pl. I, fig. 11, b). The eye of Bumastus also is environed by a similar zone, with a structure like that of the eye (Pl. II fig 35, 41).

In an undetermined species of Asaphus the lenses, although somewhat apart, are of an elongated hexaedric outline, which passes into a regular circular one farther away and on the surface of the eye they are slightly convex (Pl. I figs 27-29). In other genera belonging to this group the shape of the lenses are like those of Asaphus, so for instance in Illænus (I. chiron and I. Esmarki) and in Niobe. In Dysplanus centrotus they are shorter and broad, and their interior or lower surface strongly convex. It is likewise so in Nileus, where Nileus armadillo has an exceedingly thick exterior integument above the lenses. Such an integument has in a still higher degree increased in Bumastus sulcatus so as to exceed in thickness the stratum of the corneal prisms and it may in fact be doubted if the eyes of this species ever were able to function as visual organs. Proetus nearly resembles Bumastus in the thickness of the integument covering the prismatic lenses, which are interiorly convex, with a diameter of 0,03 mm.