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.
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.
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 and Leidig, but the figures of the distribution of the ridges are worthless. The description of Professor Forbes 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
- American Naturalist, 1872, Plate II, Figs. 1 and 2.
- Untersuchungen z. Anatomie und Histologie d. Tiere, Plate III, Fig. 28.
- American Naturalist, 16, 1882, p. 2.