THE ORIGIN OF VERTEBRATED ANIMALS 175 



as in the snakes. Among reptiles we find lizards, 

 crocodiles, turtles, and snakes ; and many extinct 

 groups are known. 



d. Aves, or birds, are characterised by the posses- 

 sion of wings and feathers, and have special modifi- 

 cations of the skeleton to aid in flight. 



e. Mammalia. — These are typically terrestrial 

 animals. Their main characteristics are the posses- 

 sion of hair, the presence of two pairs of limbs, 

 furnished with claws or hoofs, and the fact that they 

 do not lay eggs, but give birth to young which are 

 suckled by milk glands. 



II. Acrania. — This group contains a number of 

 lowly organised forms, possessing no skull, no dis- 

 tinct brain, and no limbs. Their sense-organs and 

 other parts, such as the heart and liver, are in a 

 very primitive and simple condition. They are 

 interesting as telling us in which direction to look 

 for ancestors, for they show us that these may be 

 destitute of limbs, eyes, ears, or backbone, or even 

 without any skeleton. I propose to leave this group 

 for a time, and first consider in more detail the 

 several groups of Craniota. 



Pisces. — The characteristic mode of breathing in 

 fish is by gills. Gill-clefts have no meaning apart 

 from their respiratory function, yet they are present 

 in the early stages of development of all Vertebrates 

 without exception. They are one of the most charac- 

 teristic features of Vertebrates, and have a constant 

 relation to the heart, blood-vessels, nerves, muscles, 

 and skeleton. In fish and some Amphibia they 

 are preserved in a functional state throughout life. 



