II CHARACTERISTICS OF VALUABLE MARINE FISHES 35 



a backbone or some rudiment thereof, which breathe water bv 

 means of openings from the throat behind the mouth, and which 

 usually possess paired and unpaired fins. The gills of fishes are in 

 most cases rows of delicate ridges or of projections on the sides of 

 the gill openings (G. S., Fig. i.) ; the blood flows through these 

 ridges or projections, and is thus exposed to the influence of the 

 water which is constantly passed through the gill-openings from 

 the mouth. The fins are thin projecting folds of skin supported by 

 movable hard firm rods called fin-rays. The unpaired fins are 

 in the middle line between the two similar halves of the body, 

 there are some on the back called the dorsal fins, some on the 

 belly called the ventral fins, and one at the hinder end of the body, 

 namely, the tail-fin. The paired fins are on the sides of the 

 body, and attached by a short base. There are never more than 

 two pairs, corresponding to our own arms and legs, or to the 

 forelegs and hindlegs of a quadruped. The fin-rays in fact 

 represent in a general way the toes or fingers of the limbs of the 

 higher animals, while in fishes the long limb projecting from 

 the body is not required, and is represented only by the bone or 

 cartilage at the base of the fin. The front pair of fins corre- 

 sponding to the arms of the human body are always placed close 

 behind the gills and may be called the breast-fins ox pectoral fins 

 (Pc. F., Fig. I.). The second pair are placed nearer to the lower 

 edge of the body, and in many fishes are situated in their original 

 position far back near the vent, as for instance in a dog-fish or 

 salmon, while in many others they are below or even in front of 

 the breast-fins, as for instance in the mackerel or cod. These 

 fins, whatever their position, maybe called the pelvic Ji)is (Pv. F., 

 Fig. I.), while they are sometimes called abdominal fins or throat 

 fins according to the part of the body in which they are situated. 

 The organs of sense with which we are familiar in higher 

 animals, namely, nose, eyes, and ears, occur in a well-developed 

 condition in most fishes, but with certain important differences 

 in structure. The nostrils as a rule have no posterior communi- 

 cation with the throat, but are merely pits opening on to the 

 surface of the skin. The ear consists only of the essential 

 apparatus of hearing, a closed cavity hidden in the side of the 

 skull, and not connected with structures externally visible. 

 In addition to these there is generally an additional sensory 

 apparatus which is very characteristic of fishes, and which 



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