36 



THE AQUARIUM, APRIL, 1893. 



the upper and lower ends, leaving a 

 centre part run out into a point. When 

 the backbone of a fish extends into the 

 upper part of the caudal fin, for instance 

 in the sturgeon, its tail is heterocercal. 



The fins are constructed of a frame- 

 work of bony spines or cartilage and 

 these are connected with a web of skin. 

 They are exceedingly mobile and are 

 operated by especial sets of muscles, 

 their motion adding very much indeed 

 to the graceful appearance of the fish. 

 If the spines supporting the web are of 

 bone, the fin is then called a spinous 

 fin ; while those composed of cartilage 

 and which are branching and articulate 

 are known as soft fins. The adipose 

 fin which is found back of the dorsal 

 fin on salmon and catfish, is not j^roperly 

 a fin, being nothing more than a pecu- 

 liar fatty fin-like projection from the 

 back, though it is a distinguishing mark 

 characterizing the families to which it 

 belongs. 



The water in which the fish lives is 

 very nearly as heavy as the fish itself, 

 the latter then requiring comparatively 

 but little strength to move about. 



The motion necessitating the greatest 

 expenditure of power is that of propul- 

 sion forward and is accomplished by the 

 action of the caudal fin, it being sup- 

 plied with the most powerful muscles. 

 During the act of moving forward, 

 this fin is twisted in a peculiar man- 

 ner alternately from the right to the 

 left, its action being the principle upon 

 which the screw-propeller is constructed 

 and from this the reader may gain some 

 idea of the manner in which the fin 

 operates. 



The little strength required by a fish 

 to move, readily accounts for its ability 

 to swim against the stream in rapidly 

 running waters, without any observable 

 fatigue. 



The pectoral fins are used to change 

 the water in the neighborhood of the 

 gills, thus serving as adjunct respira- 

 tory organs. The ventral fins are mainly 

 useful to regulate upward or downward 

 motions and as a brake when the fish 

 wishes to come to a complete stop when 

 in motion. The dorsal and anal fins 

 serve the purjaose of balancing the body, 

 just as the masts and keel of a ship do; 

 the former fin if spinous is also a weapon 

 of offense and defence. 



We come now to the consideration of 

 the respiratory or breathing process and 

 the organs by which it is carried on. In 

 this particular, fish differ very much 

 from other vertebrates, in this that the 

 breathing is done through the medium 

 of gills instead of lungs and the life 

 giving principle is absorbed from the 

 water instead of directly from the at- 

 mosphere. 



The gills, which are the organs of 

 respiration, are situated on each side of 

 the fish's head and consist of bony 

 arclies, which are covered with a tissue 

 that contains innumerable blood-ves- 

 sels. As a rule there are four of such 

 arches on each side of the head. 



These gills are so connected with the 

 mouth of the fish, that it is enabled to 

 press all or a part of the water taken 

 into the mouth through them. 



The life-giving principle which sup- 

 ports all animal life is a gas called oxy- 

 gen, and as this exists in the water, 

 the function of the gills is to extract it. 

 When the water passes through them 

 it comes in contact with the blood-ves- 

 sels and the oxygen is absorbed into the 

 blood, for the nourishment of that fluid 

 and the body generally. 



It will thus be seen that it is not the 

 water that the fish breathe, as many 

 suppose, but the air that is contained 

 in it, as can be proved by placing fish 



