VERTEBRATES — CLARK 287 



and marine, can live for a considerable time out of water, and some 

 fresh-water fishes, as the common eel, occasionally or regularly pass 

 overland from one body of water to another at night. In contrast 

 to amphibians, fishes are both fresh-water and marine, many groups 

 being represented both in the sea and in lakes, ponds, and streams. 

 Many fishes, as salmon, shad, and sturgeon, live in the sea but run 

 up streams to spawn. These also have relatives living permanently 

 in fresh water. A few fishes, as the common eel, some gobies, and 

 some galaxiids, live in fresh water but spawn in the sea. Many 

 marine fishes freely enter fresh water, as do some porpoises, the white 

 whale, and some seals, and some will breed there if cut off from the 

 sea, as for instance the shark and the sawfish in Lake Nicaragua. 

 Some fresh-water fishes freely enter brackish or even salt water 

 regularly or occasionally, like the northern trout and the pike. Some 

 fishes, like the sticklebacks, are especially characteristic of brackish 

 water. 



Fishes are predominantly carnivorous, the smaller fresh-water 

 species feeding mostly on aquatic insects with such other invertebrates 

 as may be available, the larger on other fishes with an occasional 

 amphibian or land vertebrate. In the larger bodies of fresh water 

 a very few feed on plankton, especially when young. A few fishes 

 feed largely on detritus, some are omnivorous, and a very few are 

 vegetarian. A small number, as Myxine and some very small cat- 

 fishes, are parasitic, and in the ceratioid fishes the diminutive and 

 defective males are attached to the females as external parasites. 

 These are the only parasites among the vertebrates. 



Fishes may be scaled, completely or partly, or naked. Some have 

 large, usually scattered, dermal scutes. 



The limbs are modified into paired fins of which the posterior 

 pair may be situated anywhere between the anus and throat, modified 

 as a sucker, or absent ; rarely both pairs are absent as in the snakes 

 and some lizards. 



Breathing is almost entirely by covered gills, but three fresh- 

 water types, in South America, Africa, and Australia, have func- 

 tional lungs and a number develop internal structures that function 

 as lungs in addition to gills. A number of fishes will drown if de- 

 prived of access to air. An air bladder is commonly present. 



The body temperature is not controlled but approaches that of 

 the surroundings. Some fishes become torpid and feed but little 

 during the winter. Others bury themselves in the mud at the bottom 

 of ponds. One in Alaska and Siberia (Dallia) remains frozen in 

 solid ice through the cold season. A number of tropical fishes 

 estivate in the mud in the bottom of dried-up bodies of water, usually 

 in a mud cocoon like some amphibians. Fresh-water fishes are much 

 less ensitive to temperature changes than marine fishes. 



