334 METAZOAN PHYLA 



lower vertebrates fertilization takes place in the water, both egg cells 

 and sperm cells being passed out of the body of the parents. In terrestrial 

 vertebrates, however, internal fertilization is the rule. Internal fertiliza- 

 tion is usually accompanied by egg laying, and the young develop outside 

 the body. Thus these forms are oviparous. In all groups of vertebrates, 

 however, occur those animals which are viviparous; and the mammals, 

 almost without exception, possess that character. 



355. Advances Shown by Vertebrates. — The advances shown by the 

 chordates, as enumerated in Chap. XLVIII, give to them advantages 

 over all other animals, and in the highest vertebrates there result the 

 most efficient types of animal life this earth has yet known. As a 

 group the vertebrates are the largest and most powerful of animal types; 

 they are the most complex in structure and at the same time their 

 functions are most perfectly coordinated; and their sense organs, if 

 not the most varied, are at least the most effective in furnishing to thej^ 

 possessors knowledge of their environments. Far ahead of any other 

 system in the extent to which its development has been carried is the 

 nervous system the functioning of which is such as to lift the higher 

 vertebrates much above all other animals. 



356. Classification. — Vertebrates are divided into seven classes as 

 follows : 



1. Cyclostomata (si klo sto' ma ta; G., kyklos, circle, and stomatos, 

 mouth). — Roundmouths, including hagfishes and lampreys. 



2. Elasmobranchii (e las mo bran' ki i; G., elasmos, metal plate, 

 and L., hranchia, gill). — Including sharks, skates, and rays. 



3. Pisces (pis' ses; L., pisces, fishes). — Fishes, including all ganoids, 

 teleosts, and lungfishes. 



4. Amphibia (am fib' i a; G., amphihios, leading a double life). — 

 Salamanders, frogs, and toads. 



5. Reptilia (rep til' i a; L., reptilis, creeping). — Lizards, snakes, 

 turtles, and crocodiles. 



6. Aves (a' vez; L., aves, birds). — Birds. 



7. Mammalia (ma ma' li a; L., matnmalis, of the breast). — Mammals. 



