GENERAL CHARACTERS. 321 



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

 communicating 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 embryo. 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 covered 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 embryonic 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 surface 

 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 

 atmospheric 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 

 oxygenate 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 lung- 

 fishes it assumes the characters and functions of the lungs of the higher Verte- 

 brates, to which, indeed, it corresponds. 



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

 as in the 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 fertilised 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 respect. 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 cat-fish 

 (Aspredo), where they are pressed into the skin of the under surface of the body, 



VOL. v. 21 



