Since, however, the bleached condition, which in the first instance is an individual reaction to the absence of light, has become hereditarily established in Amblyopsis so that the bleaching goes on even when the young are reared in the light, it is evident that in Amblyopsis we have the direct effect of the environment on the individual hereditarily established.
The Eyes of the Amblyopsidæ.—The structure of the eyes has formed the basis of a separate, fully illustrated paper.[A] The prominent features in the eyes of the various species must, however, be known before the question of the origin of these forms and the causes of degeneration can be seriously considered. The eyes of the species of Chologaster are normally formed, possessing a lens, pupil, vitreous body, retina, and optic nerve, and all the eye muscles normal to the fishes. The eyes are functional. The retina is, however, very much simplified. The eye of papilliferus is, in this respect, more perfect than the eye of cornutus. In papilliferus the outer nuclear layer consists of two series of nuclei, the inner layer of about five series of nuclei, and the ganglionic layer of a complete single layer of nuclei except where the optic fibers pass between them, for an optic-fiber layer is not present. In Chologaster cornutus the outer nuclear layer has been reduced to one or two series, and the ganglionic layer to cells widely separated from each other or in rows and little groups, but no longer forming a complete layer. In Amblyopsis and Typhlichthys the largest eyes are not more than one twentieth the diameter of those of Chologaster, or one thousandth of their bulk; the lens is nearly, if not quite, obliterated; the same is true of the vitreous body and the optic nerve in the adult. Beyond this the eyes differ much. In Amblyopsis scleral cartilages are present and prominent, the pigmented layer is prominent, the outer and inner nuclear layers form one layer only, two or three cells deep. In T. subterraneus the pigmented layer is insignificant, and no pigment is ever found in it, while the outer and inner nuclear layers are still separate. In both these species the ganglionic layer forms a central core of cells. In Amblyopsis several or all the eye muscles are present; in Typhlichthys nothing is left of them.
[A] Archiv f. Entwickelungsmechanik, viii, pp. 545–617, Plates XI-XV.
Scleral cartilages are not present in Chologaster or Typhlichthys; in Troglichthys they are very prominent, sometimes several times as long as the eye. While there is no pigment left in Typhlichthys, there is in Troglichthys. The eye in the former is about 0.168 millimetre in diameter, while the entire eye of the latter is but about 0.050 millimetre, or less than one third the diameter, and less than one ninth the bulk.
The entire eye of Troglichthys is smaller than many single cells, and I shall be pardoned for not going into the details of its structure here.
Fig. 5.—Three views of the head of an Amblyopsis, prepared to show the tactile ridges.
Fig. 6.—Snout of Chologaster papilliferus, to show the tactile ridges.
The Tactile Organs.—The tactile organs are among the most important in the consideration of the blind forms. Their minute structure will form the basis of a separate paper. The prominent tactile organs about the head of Amblyopsis have been mentioned by nearly every writer, and they have been figured by Putnam-Wyman[B] and Leidig,[C] but the figures of the distribution of the ridges are worthless. The description of Professor Forbes[D] of Chologaster papilliferus is the only systematic enumeration of the ridges that has appeared. The accompanying figures, drawn by me with the camera lucida, and verified and copied by Mr. U. O. Cox, give the exact extent and position of the ridges in Amblyopsis and Chologaster papilliferus. It will be seen that in the number and distribution of the tactile area the two forms agree very closely, the eyed form having the same number and distribution of ridges or rows that the blind forms have. In Chologaster papilliferus most of the ridges are much less prominent than in the blind species, being sunk into the skin. About the nose and chin, however, the ridges are as prominent as in the other species. In the small Chologaster cornutus there are no distinct ridges at all, the tactile organs being arranged as in other species of fishes. In specimens of the same size the papillæ are not more prominent in papilliferus than in cornutus. It is only in the oldest of papilliferus that the papillæ become prominent. The number of individual papillæ in each tactile ridge differs considerably with age (size), so that an exact comparison between the large Amblyopsis and the much smaller species of Chologaster and Typhlichthys can not be made. From a number of counts made by Professor Cox I take the liberty of giving the following: Ridge No. 6 contains, in Chologaster papilliferus, six organs; in Typhlichthys, eleven; in two specimens of Amblyopsis, respectively eighty-three and one hundred and six inches long, twelve and twenty.