GENERAL CHARACTERISTICS 2689 



The gills of sharks, rays, and chimaeroids are contained in pouches, usually 

 five in number on each side; each pouch opening externally by a slit, and also com- 

 municating by a separate aperture with the pharynx. In the embryos filamentous 

 gills protrude externally from the slits. The spiracles found on the top of the head 

 in the groups mentioned are the external openings of a canal leading on each side 

 into the pharynx, and represent what is known as the first visceral cleft in the em- 

 bryo. In the bony fishes the gills, which are generally four in number, lie in one 

 undivided cavity on each side of the head, supported by their gill arches, and cov- 

 ered over by the gill cover, which is open behind. In ordinary respiration the 

 water is taken in by the mouth, and by a kind of swallowing action driven over the 

 gills, after which it is expelled by the gill opening. Allusion must be made in this 

 place to certain organs known as false gills, or pseudobranchice. These are remains 

 of a gill situated in front of the persistent ones, which was functional during em- 

 bryonic life, but in the adult appears merely as a plexus of blood vessels. Although 

 the majority of fishes breathe in the manner indicated above, in many forms this is 

 by no means the sole method of respiration. On this subject Day observes that 

 respiration in fishes is carried out normally, either by their using the air which is 

 held in water to oxygenate the blood at the gills, or "by taking in atmospheric air 

 direct, which is employed at a special organ, where it oxygenates the blood, which 

 can be returned for use into the general circulation without going through the gills. 

 The true amphibious fishes respire by the latter method. No doubt we observe that 

 fishes which normally oxygenate their blood solely at their gills do rise to the sur- 

 face in very hot weather, when the water is foul or insufficiently charged with air, 

 and take in air by the mouth; likewise we find that those which mainly take in at- 

 mospheric air direct by the mouth may, to a certain extent, be able to use their 

 gills. If fishes having these two different modes of respiration are placed in a globe 

 of water, across which a diaphragm of net is inserted below the surface, so as to 

 prevent their obtaining access to the atmosphere, those of the class which oxygen- 

 ate their blood at the gills are unaffected, whereas those which have accessory 

 breathing organs and take in air direct die from blood poisoning. ' ' 



One of the most characteristic organs of fishes is the air bladder, which is a 

 long sac filled with gas lying in the abdominal cavity, which may be either com- 

 pletely closed, or may communicate with the alimentary tract by means of a duct. 

 As it is susceptible of compression, its usual function appears to be to regulate the 

 specific gravity, or to change the centre of gravity of the fish; but in the lungfishes 

 it assumes the characteristics and functions of the lungs of the higher Vertebrates, 

 to which, indeed, it corresponds. 



Although in the few fishes which produce living voung, as well as 

 Reproduction . 



in tne sharks and rays, an actual connection takes place between the 



two sexes, in the great majority of the class the ova are deposited by the female, 

 after which they are fertilized by the male. The bony fishes lay numerous eggs 

 which are of relatively-small size and may be extremely minute, those of the eel 

 being almost microscopic; but there is a considerable degree of variation in this re- 

 spect. In the herring the number of eggs in the "roe" has been estimated at 

 twenty-five thousand, and in the cod at over nine millions. Only in a catfish 

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