THE AQUARIUM, APRIL, 1893. 



37 



in water from whicli the air has been 

 taken by prolonged boiling or other- 

 wise. 



Also, fish that are kept in a vessel 

 will come up to the surface where the 

 air can be mixed with the water when 

 that in the water of the vessel has been 

 exhausted. 



It would seem from the fact that the 

 fish breathes only the air and not the 

 water, that it could just as well live in 

 the open air where it could get j^lenty. 

 Such is not the case, however, for the 

 gills are so constituted by nature that 

 they need something to keep them 

 apart so that their surfaces may be ex- 

 posed and perform their function pro- 

 perly : they would close together and 

 the blood-vessels would cease to absorb 

 the oxygen, resulting, of course, in the 

 death of the fish. It is necessary then 

 that a stream of water should constantly 

 flow through them, as it does, the ab- 

 sorption of oxygen then going on as na- 

 ture intended. 



In some species, however, the gills 

 are so arranged that they do not stick 

 together or dry up when the fish is 

 taken out of the water, neither is the 

 tissue containing the blood vessels so 

 tender. This is true of the carj), 

 making it possible to preserve it alive 

 out of the water for days and without 

 injury to the fish, the only precaution 

 necessary is to keep the gills moistened 

 with wet moss. Just the contrary is 

 the case with the herring, which, when 

 taken out of the water, dies almost in- 

 stantly. 



The heart of a fish lies just behind 

 the head and between the gills. It is a 

 muscular organ, consisting of three 

 parts, an auricle, a ventricle, and an 

 arterial bulb. 



The venous or stale blood is pumped 

 into the gills by the heart, where it re- 



ceives a fresh supply of oxygen. From 

 the gills it .is sent to an arterial trunk 

 lying along the under side of the verti- 

 bral column, from which it is distrib- 

 uted all over the body of the fish. 



As fish have no lungs they cannot 

 possess a voice ; the sounds made by a 

 few fish, as in the drummer, for in- 

 stance, are caused by the air when it 

 leaves the air bladder. Some years 

 ago the Poindar Science Monthly con- 

 tained a paragraph, stating that the 

 fish called the " maisir " is capable of 

 uttering several distinct sounds, but 

 this may only be conjecture. 



Most fishes are supplied with an air 

 bladder, which can be filled or emptied 

 at will. 



This bladder is a sac formed of a 

 tough membrane, situated between the 

 spinal column and the stomach. It 

 varies in shape in different species, and 

 is also subdivided. 



Those species destitute of the air 

 bladder live at the bottom of the water 

 and can only rise to the surface by the 

 action of their fins. The air bladder 

 then appears to be for the purpose of 

 either increasing the weight of the fish 

 when it is empty or decreasing it when 

 full, thus exercising a modifying in- 

 fluence upon the weight of the fish when 

 compared with that of the water. 



In many cases the jaws of the fish 

 are armed with teeth, and sometimes 

 the tongue also, as in the piscivorous 

 species, where the entire mouth, the 

 tongue included, is covered with teeth. 

 Those species that have none in the 

 mouth are generally provided with 

 pharyngeal teeth ; these are bony tuber- 

 cles on the pharyngeal bones or plates 

 situated in the throat. 



The function of the eyes differs 

 greatly in different species ; as a rule 

 those fish having barbies on the lips 



