188 PHYSIOLOGY 



The development of the alimentary canal depends 

 upon the character of the food that is eaten and the 

 length of time that is necessary for its digestion. In all 

 forms it is a tube open at both ends to the outside, with 

 one or more definite enlargements. It is too long to pass 

 directly through the body and is therefore much coiled. 



Reproduction is strictly sexual and the sexes are 

 usually separate. The excretory ducts usually act as 

 ducts for the reproductive organs. 



Classification. — The vertebrates are divided into: 



Pisces or fish; aquatic forms breathing by gills, 

 usually covered with scales. The venous heart is divided 

 into an auricle and a ventricle. 



Amphibia, e.g. frogs; aquatic forms usually hav- 

 ing true feet. The heart is divided into two auricles and 

 a ventricle. A metamorphosis is common. Bushy, 

 external gills are present in the young; lungs in the 

 adult. These may persist together or they may succeed 

 each other. 



Reptilia, e.g. snakes, turtles ; having strongly ossified 

 skeleton and cornified skin. The heart is divided into 

 two auricles and two incompletely separated ventricles. 

 They have no functional gills. 



Aves or birds; warm-blooded animals having wings 

 and feathers. The four-chambered heart is completely 

 divided into right and left halves. 



Mammals, e.g. cows, dogs, human beings; warm- 

 blooded animals with hairy skin, four-chambered heart, 

 highly-developed teeth having roots. Very low forms 

 lay eggs; others bring forth their young alive. The 

 young are nourished by milk. 



