INTRODUCTORY 



241 



termed, the pharynx. This connection between the nutritive 

 apparatus and the respiratory organs is highly characteristic of 

 Vertebrates, and when it exists in other animals is considered 

 to be conclusive evidence that they are related by descent to 

 the true Vertebrates, that they are in fact more or less remote 

 cousins of the latter tribe, owing the common possession of thi9 

 and certain other characters to descent from some common 

 ancestor which existed at an early period of evolution. The 

 gills themselves are ridges of the membrane on the anterior 

 and posterior faces of the septa ; there are four of these septa 



Fig. 22.— Skull of Salmon, op. opercular bones ; pm. premaxilla ; mx- 

 maxilla. 



with a series of gill-folds in front and behind ; there is also a 

 series on the posterior face of the hyoid but none on the an- 

 terior face of the last gill-arch behind the last cleft. In front 

 of the hyoid is a reduced cleft called the spiracle. In the bony 

 fish dermal bones are developed in front of the original upper 

 jaw, on the lower jaw, and on the hyoid arch. Those in front 

 of the mouth, called the maxillae and premaxillas, form a new 

 upper jaw in front of the old one, and bear teeth. In the lung- 

 fishes, this new jaw divides the aperture of the organ of smell 

 into two external nostrils in front of the mouth and posterior 

 nostrils inside the mouth, an arrangement which persists in 

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