PISCES. 35 



This condition is permanent in the Selachii and most 

 ganoids, but in the bony fishes the lower lobe grows out 

 equal to the other (the tail becomes homocercal (p. 9), 

 although the skeleton shows a bent back-bone (Fig. 13, 

 CD)). 



The nasal sacs are two, although occasionally four nos- 

 trils are present. In no case is there a passage through 

 them to the mouth-cavity, although it is interesting "to 

 note that in the skates a groove leads back from each of 

 these organs to the mouth, recalling a transitory condition 

 in the young of higher forms. 



The gill-slits start as paired outpushings from the throat, 

 which later break through to the exterior. These may all 

 retain their separate external openings, or they may be cov- 

 ered up by a fold from the back side of the head growing 

 over them and forming an operculum. Water taken in 

 through the mouth is forced out through these slits, and is 

 thus brought in close contact with the thin-walled gills 

 lining their sides. 



In many forms an air-bladder occurs. This starts as an 

 outgrowth from the dorsal wall of the oesophagus or gullet, 

 and in many this connection persists throughout life 

 (Physostomi), but in others the duct is closed later. The 

 bladder serves as a hydrostatic apparatus, and when it is 

 expanded the specific gravity of the fish is lessened and the 

 animal can rise, while when it is compressed the animal 

 sinks. In some forms the bladder is used in producing a 

 noise. 



In all fishes the heart, situated in a pericardial chamber, 

 consists of two portions: an auricle, which receives the 

 blood returning from the body, and a ventricle, which 

 forces it forward through the gills to all parts of the animal. 

 In leaving the heart proper the blood first passes through 



