n CHARACTERISTICS OF VALUABLE MARINE FISHES 39 



and ends in a free edge behind. In the gill-cavity are the gills, 

 which are four double fringes of red processes attached to four 

 slender columns consisting chiefly of bone. These columns form 

 part of the sides of the throat, and they are separated from one 

 another by five slits, through which the water passes from the 

 throat over the gills into the gill-chamber, and then out at the 

 gill-opening. The slits correspond to those of the dog-fish, and 

 the columns supporting the gills to the partitions between the 

 slits in the dog-fish. 



Between the gristly fishes, namely sharks and rays, and the 

 familiar bony fishes there is a tribe of fishes which have some 

 of the characters of both. They form all together a wonderful 

 series of steps from the structure of one set to that of the other, 

 but are on the whole nearest in characters to the first division of 

 the true bony fishes, that is to such fishes as salmon or herring. 

 The only one to be mentioned here is the sturgeon. This fish 

 has a gill-chamber and gill-cover like a bony fish, but it has not 

 scales of the ordinary kind. In their place the skin is provided 

 with large plates of true bone in shape like large scales, but 

 much thicker. These form a row along the ridge of the back, 

 and two rows along each side. The fin-rays are all of the soft 

 kind, that is to say, composed of numerous joints and therefore 

 flexible, except the first of the breast-fin which is thick and rigid. 

 The second pair of side fins are far back. There is one dorsal 

 fin, quite short and small, placed behind those just mentioned. 

 The tail, like that of a dog-fish, has a long upper lobe, into which 

 the back-bone is continued. The snout is much produced, and 

 rather broad and flat : underneath it in front of the mouth are 

 four slender fleshy barbels. The mouth is rather small and 

 round, and the jaws are capable of being protruded consider- 

 ibly. The sturgeon grows sometimes to eighteen feet in length, 

 md is frequently taken in the North Sea, or at the mouths of our 

 large rivers. It occurs in greater numbers in the river Volga, 

 and is found also on the east coast of America. Several other 

 species of sturgeon are known, and they all spawn in large rivers 

 md descend to the sea only for a time in winter. 



Among the creatures which we distinguish in the first place 

 generally as fishes, and which resemble one another, and differ 

 )m other animals in the characters which have been mentioned 

 ibove, there are, as every one knows.a multitude of different kinds, 



