THE STRUCTURE OF BLIND FISHES. 53 



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 Ty- 

 phlichthys nothing is left of them. 



Scleral cartilages are not present in Chologaster or Typhlich- 

 thys; in Troglichthys they are very prominent, sometimes sev- 

 eral 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 diam- 

 eter, 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 struc- 

 ture 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 



Fig. 5. — Three views of the head of an Amblyopsis, prepared to show the tactile ridges. 



by nearly every writer, and they have been figured by Putnam- 

 Wyman * and Leidigyf 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 Amblyop- 

 sis and Chologaster papilliferus. It will be seen that in the num- 

 ber 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. 



f Untersuchungen z. Anatomie und Histologic d. Tiere, Plate IN, Fig. 28. 



% American Naturalist, 16, 1882, p. 2. 



