RUDIMENTARY EYES 725 



Among Invertebrates with such degenerate eyes the Isopod Typhlocirolana — a 

 small Crustacean found in a cave in the island of Majorca — may be taken as an 

 example. The compound eyes are minute degenerate bodies \ mm. in diameter, 

 without pigment in the ommatidia, while the crystalline cone and the proximal part 

 of the retina are grossly atrophied (Menacho, 1913). 



CAVE-FISHES ^ are all Teleosteans and it would seem probable that the 

 ancestors of most of them can be traced from species in which a pre- 

 adaptation to ocular regression had already been present owing to a previous 

 existence in deep seas or muddy bottoms^; few of them {e.g., catfishes of the 

 genus, Rhamdia) have well-formed eyes ; and some types {e.g., the Mexican 

 catfish, Anopticlithys jordani) show all grades of reduction of the eye from 

 normal organs to rudimentary remnants. The latter are hatched with small 

 but complete eyes, lacking, however, a circulation, and as the fish matures 

 these gradually degenerate until all that is left in the adult is a most rudi- 

 mentary organ lying deeply buried in a recognizable orbit associated with 

 hyj3oplasia of the optic lobes (Gresser and Breder, 1940-41 ; Breder, 1942 ; 

 Liiling, 1953-55 ; Kuhn and Kahling, 1954 ; Stefanilli, 1954). Some of 

 the cave-fishes derive from deep-sea types such as the Brotulidse which 

 emigrated to the surface and there sought the darkness of crevices in reefs or 

 caves. Three species have made the still more remarkable transition to 

 fresh water — Lucifuga and Stygicola which are found in caves in Cuba, and 

 TyjMias in Yucatan. Eigenmann (1909) concluded that these Cuban fishes 

 initially inhabited caves in the coral beaches where they remained as these 

 caves were elevated and became filled with and enlarged by fresh water ; 

 in his view the fishes are older than the island of Cuba. The eyes, which lie 

 under the skin, are best developed before birth; thereafter they progressively 

 degenerate until in old age they are represented by a shrivelled, pigmented 

 vesicle, lying deeply in the large orbit, a process perhaps determined by a 

 disturbance of the circulation. The bottom-grubbing catfishes which 

 habitually shun the light are the ancestors of other types. These Siluroids 

 which encyst themselves in the mud often have rudimentary eyes (Cope, 

 1864) ; thus the eye of the bull-head catfish, Amevurus, has an ill-formed 

 lens and a retina wherein the rods are large, the cones few and small, while 

 the outer nuclear layer is represented by only two rows of nuclei, the inner 

 by one, and the ganglion cells by a few widely-scattered elements. 



The Aniblyopsidfe, the North American group of cave-fishes characteristic of the 

 caves of the Mississippi basin, are of considerable interest (Telkampf, 1844 ; Wyman, 

 1850-54 ; Kohl, 1892-93 ; Eigenmami, 1899-1909 ; Hubbs, 1938). They are 



1 A monograph by Carl H. Eigenmann, the Professor of Zoology of Indiana University 

 gives a good account of the Cave-Vertebrates of America (Carnegie Inst., Washington, 1909), 

 including a particularly illuminating and interesting studj^ of the cave-fishes of the Mississippi 

 Valley and Cuba. A subsequent monograph by Hubbs, Fishes from the Caves of Yucatan 

 {Carnegie Inst. Wash., Pub. No. 491, pp. 261-295. 1938), lists all known blind fishes apart from 

 deep-sea types. 



^ Anoptichthys is an exception in that it probably entered cave life as a stray and on losing 

 its vision was constrained to remain. 



