THE ORIGIN OF MAN 33 



and fins. The gills are on each side of the head 

 and consist of delicate, hollow filaments in which 

 the blood circulates, and which are supported by 

 internal gill arches of either cartilage or bone. 

 Water is taken in through the mouth and passed 

 out over these breathing organs, where the blood 

 in the tubes extracts the free oxygen dissolved in 

 it. The external gill openings may remain open 

 and unprotected or may be covered over by a 

 hinged bony cover, the operculum. The mouth 

 may be on the under side of the head or at its for- 

 ward end, and is usually provided with teeth. 

 Also an air bladder is generally present and 

 serves as a hydrostatic organ or float, though in 

 certain forms it acts as a lung and assists the 

 gills in respiration. The skin is either soft and 

 naked, or more commonly, protected by the de- 

 velopment of spines, overlapping scales, or bony 

 plates known as scutes. The principal organ of 

 locomotion is the powerful and very flexible mus- 

 cular tail, which is assisted by the paired fins, 



