CHAPTER XII 

 CONDITIONS OF LIFE 



Deep-sea fishes. Cave-dwelling fishes: Kentucky Blind-fish, Cuban Blind- 

 fishes. Evolution of cave faunas. Californian Blind Goby. Sponge- 

 inhabiting Gobies. Hill-stream fishes and their modifications. Eflfects 

 of temperature on fishes. Catastrophe of Tile-fish. Hibernation and 

 aestivation. Association of Pilot-fish and Shark. Commensalism: 

 Remora and Shark, Pomacentrid and anemone, Rudder-fish and 

 jelly-fish, Fierasfer and echinoderm or oyster. Symbiosis. Parasitism: 

 Candiru. 



In describing the various organs of a fish's body, the relation 

 between the environment or conditions of Hfe and the structural 

 modifications has been stressed throughout. Just as the struggle 

 for existence in terrestrial regions has led to the colonisation of 

 the air by the birds, and the return to the sea by the whales 

 and other aquatic mammals, so, under the stress of competition, 

 certain fishes have been compelled to penetrate into regions 

 where it would seem impossible for them to survive, or to adopt 

 some markedly unusual mode of life. In the present chapter 

 some of the more interesting of these specialised forms may 

 be considered, and their bodily peculiarities described. 



The middle layers and abyssal depths of the oceans provide 

 a number of special physical conditions which are clearly 

 reflected in the structural modifications of the fishes inhabiting 

 these regions. The great pressure under which many of them 

 live, for example, has led to a marked reduction in the skeletal 

 and muscular systems, these parts being but feebly developed 

 as compared with the same structures in littoral forms. The 

 bones are very thin, light, frequently quite flexible, and the 

 ligaments connecting them are fragile and easily torn. The 

 lateral muscles of the trunk and tail, although powerful enough, 

 are often extremely thin, and the connective tissue binding 

 them together loose and feeble. The skin may be little more 

 than a fine membrane, and capable of great distension. Other 

 characteristic modifications involving the light-producing organs, 

 muciferous system, barbels, eyes, teeth, colour, and so on, have 

 been adequately considered in earlier chapters. 



230 



