Class 2. Pisces. 375 



In Cyclostomes and Selachians, structures corresponding with the 

 lungs of the higher Vertebrata are wanting. On the other hand, 

 a true lung, which is not only homologous with that of Amphibia 

 and others, but is actually functional as a respiratory 

 organ, occurs in some bony Ganoids {Lepidosieus and Amia), 

 in the Dipnoans, and also in a few Teleosteans. This lung is 

 unpaired or incompletely divided into two ; it lies dorsal to the 

 alimentary canal, and opens by a wide aperture into the cesophagus. 

 Within, it is furnished with folds, just as in the Prog's lung ; air 

 can be inhaled and exhaled through the mouth. These Fish have, in 

 addition to the lung, gills which also serve as respiratory organs.* 

 In the rest, a lung is also usually present; it does not act as a 

 respiratory organ, but rather as hydrostatic apparatus, 

 and is termed a swim-bladder. The swim-bladder is an 

 unpaired air-containing sac, often rather thick-walled, and situated 

 below the vertebral column, dorsal to the alimentary canal ; in many 

 Fish it communicates with the cesophagust by a long, narrow tube, 

 the pneumatic duct; in others, such a connection is present 

 only in the embryo, closing and disappearing later. The swim- 

 bladder is sometimes incompletely divided into anterior and posterior 

 portions (Carp) , by a transverse constriction ; or it may be provided 

 with evaginations. The gas contained in the swim -bladder is not 

 taken direct from the atmosphere, but is excreted from the vessels 

 lying in the walls ; they of ten form close circumscribed rotiamirabilia, 

 projecting as " red bodies " on the inner side of the bladder. 



Many Fish, e.g., the common freshwater forms, in wMct the ah'-bladder is not 

 respiratory, nevertheless come occasionally to the surface and gulp in atmospheric 

 air through the mouth ; this is soon sent out again. It probably has to do with 

 an oral respiration of subordinate impoi'tance. In a few Fish in connection 

 Tvith this, a special respiratory apparatus is developed; for instance, 

 in some Siluroids (Saccobranchits), there is a sac-like evagination on each side of 

 the oral cavity which serves as a lung. It opens into the mouth in front of 

 the first gill-bar, and extends far back into the body ; so also in a kind of Eel 

 (Amphipnous), an East Indian form, which lives in holes in the ground, and 

 whose gills are very degenerate. In the East Indian OHmbiug Perch {Anabas), 

 which often wanders on to land, and has similarly feebly- developed gills, there 

 are peculiar pleated laminae (supported by modified portions of the giU-hars), 

 which act as respiratory organs, in the upper part of the branchial chamber. 

 In others, e.g., the Loach [Gohitis), intestinal respiration occui's ; 

 air is inhaled thi-ough the mouth, and passed on to those parts of the ali- 

 mentary canal, which are specially vascular ; the air which is not absorbed 

 ■escapes from the anus, together with excreted carbonic acid gas. 



Many Pisces can produce sounds. The wall of the swim-bladder is made 

 to vibrate by the action of certain skeletal muscles which are apphed to it, 

 as in the Gurnard ; or particular bony sui-f aces may be rubbed against one 



* Some Fish can survive a drought, during which the gills are functionless for some 

 time. 



t In some, the pneumatic duct opens fiurther hack, into the stomach. 



