INTRODUCTION 



15 



among themselves than other classes of vertebrates. In some 

 respects they appear to be related to both birds and mammals. 

 They have scaly or armored skins, never have gills, but breathe by 

 lungs and are cold blooded. They have three chambers to the 

 heart, the ventricular septum being perforate in all except the 

 crocodilia. Living forms are terrestrial or aquatic but extinct forms 

 were aerial. With the exception of a few lizards and snakes, they 

 are oviparous. (Lizards, snakes, turtles, alligators.) 



Class Amphibia (4,000 species). — With slimy skins, the modern 

 species lacking armor, but externally resembling the reptiles so 

 much that Cuvier once termed them "naked reptiles," the amphibia 

 mark a transition from the aquatic life of fishes to the terrestrial 

 life of reptiles. In the larval condition practically all amphibia 

 have gills, while as adults they breathe by lungs, although in some 

 forms gills still persist. Amphibia are cold blooded, and their 

 unpaired fins are never supported by fin rays. They are, with a few 

 exceptions, oviparous. (Salamanders, frogs, toads.) 



Class Pisces (20,000 species). — As strikingly adapted to life in 

 the water as birds are to life in the air, the fishes are all aquatic, 

 moving chiefly by a muscular tail. They have paired appendages 

 in the form of fins, and unpaired median fins always supported by 

 fin rays. All have permanent gills supported by cartilaginous or 

 bony gill arches. Fishes are cold blooded, the body temperature 

 remaining the same as that of the medium in which they swim. 

 The heart is two chambered, only the lung fishes exhibiting a prim- 

 itive auricular septum. In the skin of most fishes, one finds scales. 

 Some fishes are oviparous, others are viviparous. (Shark, sturgeon, 

 mackerel, trout.) 



Class Cyclostomata. — While they somewhat resemble the bony 

 true eels, the hags and lampreys have no jaws, no lateral appendages 

 and no scales. A rasping tongue and a circular sucking mouth are 

 present. The gills are pocket-like and the vertebrae are separate 

 from the notochord. Lampreys are true vertebrate parasites. 



Sub-phylum Adelochorda {Cephalochordd). — Fish-like forms once 

 classed with the mollusca. Branchiostoma (Amphioxus) has a 

 dorsal fin, lateral metapleural folds, well developed myotomes, and 

 a persistent notochord. The nerve cord has a neurocele. A 

 pharynx, with many gill slits, leads into a ventral atrium and 

 currents pass out the atriopore. A cranium is absent. 



Sub-phylum Urochorda (1,500 species). — Once called worms, the 



