320 SYSTEMATIC ZOOLOGY. 



the folded olfactory membrane. In no case is there any 

 connection of the nasal cavities with the mouth or throat, 

 although it is interesting to note that in the skates a 

 groove leads backward from each nostril to the mouth, 

 recalling the way in which the internal nostrils are formed 

 in the young of the higher vertebrates. 



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 

 covered up by a fold from the back side of the head grow- 

 ing over them and forming an operculum. (See also p. 

 308). 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 arises 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 pro- 

 ducing 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 

 an arterial cone or an arterial bulb (fig. 121). These 

 differ in this: the arterial cone is really an outgrowth of 

 the heart, and contains, on its interior, valves to prevent 

 the flow of the blood back into the ventricle; the arterial 

 bulb, on the other hand, is merely a muscular thickening 



