FISHES AND FISHERIES 241 



is free so that water may be taken in at the mouth and pass 

 out through the gill-openings. The gills consist of a series 

 of slender filaments attached to bony arches. In these fila- 

 ments a supply of blood is constantly circulating in fine 

 capillaries through the walls of which carbon dioxide is given 

 off and a fresh supply of oxygen taken from the water that is 

 flowing over the gills. In some of the sharks and in some of 

 the flat rays that lie on the sea-bottom a pair of spiracles or small 

 openings occur behind the eyes. These open into the mouth 

 and the water can pass in through them instead of through the 

 mouth-opening. On the head of some fishes are to be found 

 soft pendulous filaments, sharp spines, or other appendages. 

 On the trunk usually occur two pairs of paired fins and two or 

 more unpaired fins. 



Just back of the gill-openings are the pectoral fins, which 

 are homologous with the fore limbs of the other vertebrates. 

 On the ventral side of the body is another pair of fins, the 

 pelvic fins, which correspond to the hind limbs. The pelvic 

 fins may be placed well forward, almost under the head, or 

 well back on the body. The unpaired or median fins consist 

 of the dorsal fin above, the anal fin below, and at the posterior 

 end of the body, the caudal fin, or tail. Salmon and trout, 

 and a few other fishes, have in addition to these a small, soft 

 adipose fin between the dorsal and caudal fins. The median 

 fins are folds of the skin of the body supported by more or less 

 firm rays. The stiff unjointed rays are known as spines and 

 the others, which are softer and made up of little joints, are 

 called soft rays. 



The stiff sharp spines in the paired median fins are often 

 very effective weapons of defense as a wound made by them 

 may be very severe, particularly when made by the spines with 

 serrated or ragged edges. Some of the scorpion-fishes and 

 others secrete a poison which is introduced into the wound 

 made by the spine. 



Along the side of most fishes, extending from the head to 

 the caudal fin, is a series of modified scales which mark the 

 lateral line. This lateral line is subject to considerable varia- 

 tion in regard to its position and structure. In connection 



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