THE LOWEST VERTEBRATES 157 



developed brain and a notochord which is not segmented. 

 The gills lie in pouches, and the water passes out through 

 one or more pairs of apertures in the sides of the body. 



Most of the cyclostomes live upon other fishes to which 

 they attach themselves by a sucker-like mouth which is 

 furnished with a rasping apparatus for abrading the flesh. 

 The lampreys are mostly fresh-water forms and the few 

 marine species migrate up rivers to breed, the eggs of 

 several species being deposited in rude nests constructed 

 out of small stones. The hag-fishes are marine; often 

 they bore through the body walls of fishes and devour 

 most of the internal flesh. 



FIG. 127. A brook lamprey. (Modified from Gage.) 



The vertebrates, in the most restricted sense of the term, 

 comprise the generally recognized classes of Cyclostomes, 

 Fishes, Amphibians or Batrachia^is, Reptiles, Birds and 

 Mammals. Originally the vertebrates were aquatic 

 animals breathing by means of gills, and the lowest classes, 

 the cyclostomes and fishes, still retain the ancient habit of 

 living in the water. The Amphibians, as the name im- 

 plies (amphi, both and bios, life) live both in water and on 

 land, the more fish-like members being aquatic gill- 

 breathers, while the higher amphibians, such as frogs and 

 toads, live upon land; but even these tend to remain near 

 the water or at least in moist surroundings. The reptiles, 

 birds and mammals have become primarily terrestrial 

 animals. 



