34 THE ORIGIN OF MAN 



of which the forward set, known as the pectoral 

 limbs, lie just behind the gills, and the rear pair, 

 or pelvic limbs, are placed farther back. Fishes 

 also usually possess a variable number of un- 

 paired fins which are used mainly as balancing 

 organs; those on the back are called dorsal fins, 

 those on the under side between the anus and tail 

 are known as anal fins, and the tail represents a 

 caudal fin. All the fins are supported by skeletal 

 bars or rays, called fin rays, and are covered by 

 muscles and skin. Fishes do not have ears and 

 therefore there is no ear drum or tympanum (ear 

 cavity) and no tympanic membrane. The main 

 nerve trunk line lies on the upper or dorsal side 

 of the vertebrae (the main skeleton consists of 

 the vertebral column made up of joints known as 

 vertebrae) which extend from the brain case to 

 the tail, and it is more or less protected by a bony 

 arch known as the neural arch. Beneath the 

 vertebral column are situated the digestive and 

 reproductive organs. A few words about the 



