In all vertebrates these pineal eyes are separated from the external surface by a greater or less thickness of tissues; in the case of Ammocoetes, as is seen in Fig. 31, the eye lies within the membranous cranial wall, and is attached closely to it. The position, then, of the cuticular, or corneal lens, as it is often called, on the supposition that this is a median eye of the arachnid type, is taken by the membranous cranium, and, as I have described in my paper in the _Quarterly Journal_, on carefully lifting the eye in the fresh condition from the cranial wall, it can be seen under a dissecting microscope that the cranial wall often forms at this spot a lens-like bulging, which fits the shallow concavity of the surface of the eye, and requires some little force to separate it from the eye. As will appear in a subsequent chapter, this cranial wall has been formed by the growth, laterally and dorsally, of a skeletal structure known by the name of the _plastron_. The last part of it to be completed would be that part in the mid-dorsal line, where apparently, in consequence of the insinking of the degenerating eyes, a dermal and subdermal layer already intervened between the source of light and the eyes themselves. When the membranous cranium was completed in the mid-dorsal region, it was situated here as elsewhere just internally to the subdermal layer, and therefore enclosed the pineal eyes. This, to my mind, is the reason why the pineal eyes, which, in all other respects, conform to the type of the median eyes of an arachnid-like animal, do not possess a cuticular lens. Other observers have attempted to make a lens out of the elongated cells of the anterior wall of the eye (my corneagen layer), but without success. Studniçka, who calls this layer the _pellucida_, does not look upon it as the lens, but considers, strangely enough, that the translucent appearances at the ends of each nerve end-cell represent a lens for that cell, so that every nerve end-cell has its own lens. Still more strange is it that, holding this view, he should yet consider these knobs {87}to be joined by filaments to the cells in the anterior wall of the eye, a conception fatal to the action of such knobs as lenses.