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 Ammocoetes (Fig. 31). Originally, as described by Scott, the eye stood vertically {79}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. [Illustration: FIG. 34.--HORIZONTAL SECTION THROUGH BRAIN OF AMMOCOETES, TO SHOW THE LEFT, OR VENTRAL PINEAL EYE. _pn._2_, left or ventral pineal eye; _pn._1_, last remnant of right, or dorsal pineal eye; _g.h.r._, right _ganglion habenulæ_; _g.h.l._1_, _g.h.l._3_, parts of left _ganglion habenulæ_; _pi._, fold of _pia mater_ which separates the left _ganglion habenulæ_ from the left pineal eye; _f._, strands of nerve-fibres connecting the left eye with its ganglion, _g.h.l._3_; _V_3_, third ventricle; _V.aq._, ventricle of aquæduct.] The second, ventral or left, eye, belonging to the left ganglion habenulæ is very different in appearance, being much less evidently an eye. Fig. 34 is one of the same series of horizontal sections as Fig. 31, _pn._1_ being the last remnant of the right, or dorsal, eye, while _pn._2_ shows the left, or ventral, eye with its connection with the left _ganglion habenulæ_.