ANIMALS CLASSIFIED 



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secreting mucus, a slimy substance which helps them go through 

 the water easily. They usually lay very many eggs. 



Classification of Fishes 



Order I. The ElasmobrancJis. Fishes which have a soft skeleton made of cartilage 



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

 Order II. The Ganoids. Fishes which once were very numerous on the earth, but 



which are now almost extinct. They are protected by platelike scales. Ex- 

 amples : gars, sturgeon, and bowfin. 

 Order III. The Teleosts, 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 IV. The Dipnoi, or Lung Fishes. 



This is a very small group. In many A bony fish. 



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. 



Characteristics of Amphibia. — The frog belongs to the class of 

 vertebrates known as Amphibia. As the name indicates {amphi, 

 both, and bia, life), members of this group live 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 



Newt. (From a photograph loaned by the American Museum of Natural 



History.) About natural size. 



skin is soft and unprotected by bony plates or scales. The heart 

 has three chambers, two auricles and one ventricle. Most am- 

 phibians undergo a complete metamorphosis, or change of form, 

 the young being unlike the adults. 



