The Median Eyes of Ammocœtes.
The evidence of Ammocœtes is so conclusive that I, for one, cannot conceive how it is possible for any zoologist to doubt whether the parietal organ, as they insist on calling it, had ever been an eye, or rather a pair of eyes.
Anyone who examines the head of the larval lamprey will see on the dorsal side, in the median line, first, a somewhat circular orifice—the unpaired nasal opening; and then, tailwards to this, a well-marked circular spot, where the skin is distinctly more transparent than elsewhere. This spot coincides in position with the underlying dorsal pineal eye, which shines out conspicuously owing to the glistening whiteness of its pigment. Upon opening the brain-case the appearance as in Fig. [20] is seen, and the mass of the right ganglion habenulæ (G.H.R.), as it has been called, stands out conspicuously as well as the right or dorsal pineal eye (Pn.). Both eye and ganglion appear at first sight to be one-sided, but further examination shows that a left ganglion habenulæ is present, though much smaller than on the right side. In connection with this is another eye-like organ—the left or ventral pineal eye,—much more aborted, much less like an eye than the dorsal one; so also there are two bundles of peculiar fibres called Meynert's bundles, which connect this region with the infra-infundibular region of the brain; of these, the right Meynert's bundle is much larger than the left.
Fig. 31.—One of a Series of Horizontal Sections through the Head of Ammocœtes.
l.m., upper lip muscles; m.c., muco-cartilage; n., nose; na.c., nasal cartilage; pn., right pineal eye and nerve; g.h.r., right ganglion habenulæ; s.m., somatic muscles; cr., membranous wall of cranium; ch., choroid plexus; gl., glandular substance and pigment filling up brain-case.
| Fig. 32.—Eye of Acilius Larva, with its Optic Ganglion. On the right side the nerve end-cells have been drawn free from pigment. | Fig. 33.—Pineal Eye of Ammocœtes, with its Ganglion Habenulæ. On the left side the eye is drawn as it appeared in the section. On the right side I have removed the pigment from the nerve end-cells, and drawn the eye as, in my opinion, it would appear if it were functional. |
This difference between right and left indicates a greater degeneration on the left side, and points distinctly to a close relationship between the nerve-masses known as ganglia habenulæ and the median eyes. In my opinion this ganglion is, in part, at all events, the optic ganglion of the median eye on each side. It is built up on the same type as the optic ganglia of invertebrate simple eyes, with a cortex of small round cells and a medulla of fine nerve-fibres. Into this ganglion, on the right side, there passes a very well-defined nerve—the nerve of the dorsal eye. The eye itself with its nerve, pn., and its optic ganglion, g.h.r., is beautifully shown by means of a horizontal section through the head of Ammocœtes (Fig. [31]). Originally, as described by Scott, the eye stood vertically above its optic ganglion, and presented an appearance remarkably like Fig. [32], which represents one of the simple eyes and optic ganglia of a larva of Acilius as described by Patten; then, with the forward growth of the upper lip, the right pineal eye was dragged forward and its nerve pulled horizontally over the ganglion habenulæ. For this reason the eye, nerve, and ganglion are better shown in a nearly horizontal than in a transverse section.
The optic nerve belonging to this eye is most evident and clearly shown in Fig. [31], and in the series of consecutive sections which follow upon this section; no doubt can arise as to the structure in question having been the nerve of the eye, even though, as is possible, it does not contain any functional nerve-fibres.
