390 ADAPTATIONS TO MEDIA AND SUBSTRATES 



ably lose what remained of its eyes, though without being under any 

 positive necessity of doing so. As to whether Rhamdia spp. have only 

 just found their caves, or are simply slow mutators — the reader may take 

 his choice. 



It is perhaps worth pointing out that even an individual fish, of some 

 kinds, may be unable to retain useful eyes if kept in darkness. Ogneff, 

 thirty years ago, kept some goldfish in the dark for three years. At the 

 end of that time they had lost their skin pigment, their eyes had degen- 

 erated greatly — though not in any close imitation of those of normally- 

 blind fishes — and they were quite unresponsive to light. Conversely, it 

 has been found that in cave salamanders {Proteus, Typhlotriton) whose 

 larval eyes normally retrogress at metamorphosis to the point of obsoles- 

 cence, the eyes can become quite normal salamander eyes if the larvae 

 grow to adulthood in the light. These sightless amphibians thus become 

 blind in each new generation. No mandatory degeneration of the eyes is 

 genetically fixed in the species — merely a capacity of the whole eye to 

 retrogress if it is not used past a certain point in its development, as in 

 the case of Ogneff's goldfish. 



Whether or not the adult ocular degeneracy of any, or many, cave 

 fishes has a similar basis, is something for future experiments to decide. 

 And, the cave fishes are but one facet of the general problem of quasi- 

 eyelessness. Blind, fossorial species are to be seen in every class of verte- 

 brates except the birds. 



Parasitic Fishes — One strange habitat, which is about as lightless as 

 any, is the interior of an animal. The hordes of internally parasitic inver- 

 tebrate animals are all eyeless, with the other sense-organs, as well as the 

 organs of digestion and locomotion, greatly reduced or absent. 



A very few vertebrates, all of them fishes, are parasitic. The larger 

 lampreys are external parasites on other fishes. While clinging to a host, 

 a lamprey has little need for vision; but since lampreys ordinarily con- 

 sume only blood, they necessarily spend a good deal of time off of hosts, 

 engaged in a search for the next victim. Their eyes are important at such 

 times, for the exploration is largely visual — it has been shown that lam- 

 preys are attracted to any light-colored object (which could seem to 

 them to be a fish's belly) moving through the water. They will cling to 

 a white-bottomed boat, but not to a dark one; and lampreys have given 

 considerable trouble to human swimmers by mistaking them for fishes. 

 The eyes of lampreys (Fig. 103, p. 258) are excellent visual organs and 

 are in no way degenerate. 



