ADAPTATIONS 403 



maim the same shrimp is found all over the Bay of San Diego 

 and is accompanied by other species of goby such as Clevelandia 

 and Gillichthys ; but these fishes live outside the holes and only 

 retreat into them when frightened, while the blind species 

 is found only at Point Loma and never leaves the burrows 

 of the shrimp. In this case therefore the relations of Typhlo- 

 gobius and the other gobies of the neighbourhood are similar 

 to those of Typhlichthys and Chologaster among the fresh- 

 water fishes of the Mississippi region, and it seems more pro- 

 bable that Typhlogobius is allied to Gillichthys or some other 

 species of the same locality than to Crystallogobius. In fact, it 

 is difficult to avoid the conclusion that the blind goby is simply 

 one of the ordinary species of the district which has lost its 

 eyes and pigment in consequence of its subterranean habits. 



The development of Typhlogobius has been investigated 

 and it has been found that, as in the blind cave-fishes, the eyes 

 and pigment are much more developed in the young than in 

 the adult, degeneration taking place subsequently. In the 

 embryo of Typhlogobius within the egg the eyes and the pig- 

 ment cells in the skin are developed exactly as in any ordinary 

 species; in young specimens about -— in. long the eyes are 

 still externally visible, the membranes of the fins are thin and 

 the skin is pigmented, the movements of the fish are also quick 

 and active. In the adult the eyes are visible as minute specks, 

 but are covered over by thick skin, the membranes of the fins 

 are thick, the skin is without pigment, and the movements are 

 sluggish ; the adult fish is, however, extremely tenacious of life, 

 surviving for a long time in vessels of foul water in which all 

 other animals have died. Anatomical investigation shows that 

 the eyes of the adult are covered by a thick layer of skin while 

 the sclerotic or proper capsule of the eye is thin and connected 

 with the surrounding fibrous tissue; the lens is present and 

 spherical but the cavity behind it is collapsed and the retina is 

 rudimentary ; the optic nerve is very minute and the eye-muscles 

 rudimentary. It is evident that the eyes are functionless. 



With regard to the skin, microscopic examination showed 

 that the pigment was not entirely absent in the large speci- 

 mens. The pigment is situated on the inner surface of the 

 skin, the thickness of which in the adult fish is much increased 

 and which is also very richly supplied with blood-vessels. It 



