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 {77}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. [Illustration: FIG. 31.--ONE OF A SERIES OF HORIZONTAL SECTIONS THROUGH THE HEAD OF AMMOCOETES. _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.] {78}[Illustration: FIG. 32.--EYE OF ACILIUS LARVA, WITH ITS OPTIC GANGLION. On the right side the nerve end-cells have been drawn free from pigment.] [Illustration: FIG. 33.--PINEAL EYE OF AMMOCOETES, 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.]