246 CLASSIFICATION OF PLANTS AND ANIMALS 



Classification of Fishes 



Ordeb 1. Elasmohranfchii. Fishes which have a soft skeleton made of cartilage, and ex- 

 posed gill slits. Examples : sharks, skates, and rays. 



Order 2. Ganoi'dei. Fishes which once were very numerous on the earth, but which are 

 now almost extinct. They are protected by platelike scales. Examples : gars, sturgeon, 

 and bowfin. 



Order 3. Teleos'tei, or Bony Fishes. They compose 95 per cent of all living fishes. In this 

 group the skeleton is bony, the gills are protected by an operculum, and the eggs are 

 numerous. Most of our common food fishes belong to this class. 



Order 4. Dipnoi, or Lung Fishes. This is a very small group. In many respects they are 

 more like amphibians than fishes, the swim bladder being used as a lung. They live in 

 tropical Africa, South America, and Australia, inhabiting the rivers and lakes there. 



Amphibia. — The frog (page 230) belongs to the class of vertebrates known 

 as Amphibia. As the name indicates (amphi, both, and hios, life), members of 

 this group live during their life history both in water and on land. In the earlier 

 stages of their development they take oxygen into the blood by means of gills. 

 When adult, however, they breathe by means of lungs. At all times, but 

 especially during the winter, the skin serves as a breathing organ. The skin 

 is soft and unprotected by bony plates or scales. The heart has three cham- 

 bers: two auricles and one ventricle. (See figure, page 171.) Most amphib- 

 ians undergo a metamorphosis, or change of form, the young being unlike 

 the adults. About 1500 species are named. 



Classification of Amphibia 



Order 1. Urode'la. Amphibia having poorly developed appendages. Tail persistent 



through life. Examples : mud puppy, newt, salamander. 

 Order 2. Anu'ra. Tail-less Amphibia, which undergo a metamorphosis, breathing by gills 



in larval state, by lungs in adult state. Examples : toad and frog. 



Reptiles. — These animals are characterized by having scales developed 

 from the skin. In the turtle they have become bony and are connected with 

 the internal skeleton. Reptiles always breathe by means of lungs, differing 



Turtle 



Lizard 



Snake 

 Reptiles 



Crocodile 



in this respect from the amphibians. They show their distant relationship to 

 birds in that their large eggs are incased in a leathery, limy shell. There are 

 about 1500 known species. 



