336 ANIMALS 



Crustaceans include crayfishes, lobsters, crabs, and barnacles. 

 They have three body regions: head, thorax, and abdomen. The 

 crayfish has a one-chambered heart and arteries, but no veins. Res- 

 piration is by gills. 



Insects also have three body regions. Respiration is in air passages, 

 and not in gills. When insects undergo metamorphosis, there are 

 usually three stages: the larva, pupa, and adult stages. Insects 

 (including spiders) form the most numerous class of animals. 



Fishes and all animals above them are called vertebrates. They 

 have a spinal cord and an interior bony framework. All animals below 

 fishes are called invertebrates. The heart of the fish is two-chambered, 

 its blood is red, and its gills are well developed. 



Amphibians have limbs, and live partly on land and partly in 

 water. Frogs and toads hatch out as tadpoles. The heart of am- 

 phibians has two auricles and one ventricle. The adult breathes 

 v/ith lungs. 



Reptiles breathe only with lungs. The body is covered with horny 

 plates. The highest reptiles, the crocodiles, have a four-chambered 

 heart. 



Birds show a great advance over reptiles, but have some reptilian 

 qualities. Their blood is, however, warm. The brain and nervous 

 system are highly developed. The body is covered with feathers. 



Mammals have a covering of hair, and bring forth their young alive. 

 They are all air breathers. The brain and nervous system are the 

 highest among animals. Most mammals are either herbivorous or 

 carnivorous. Man is omnivorous; that is, he eats both vegetable and 

 animal food. 



The high development of animals is due to the " division of labor" 

 among the cells. 



Man has had a great influence in the development of economic 

 animals, and animals have had a powerful influence on the develop- 

 ment of man. 



349. Exercises. 



1. Name some invertebrates that are used as food by man. Which 

 is the most important in this respect? Are any amphibians used as 



