ii CHARACTERISTICS OF VALUABLE MARINE FISHES 45 



A great many of the fishes belonging to this division are 

 inhabitants of fresh water, and our commonest fresh-water fishes 

 belong to one family, the carp family. Many of the species pass 

 certain periods of their lives in fresh water, and at other periods 

 live in the sea. Some of these, like the salmon, ascend rivers in 

 order to spawn, and descend after spawning : these are said to be 

 anadromous, or ascending. Others, like the eel, live in rivers or 

 ponds all their lives except when about to spawn, and then 

 migrate to the sea : these are said to be katadromous, or 



FIG. 6. The Thorn back, dorsal surface. 



descending. Excluding the species which belong entirely to 

 fresh water, there are only four families in British waters which 

 are of any commercial importance, namely, the herring family, 

 the salmon family, the eel family, and the gar-fish family. 



The fishes of the herring family (Fig. 7) have a single dorsal 

 fin of no great extent, situated near the middle of the back, and 

 a single ventral fin near the tail. The tail-fin is deeply forked. 

 It is a remarkable peculiarity in these fishes that the lateral line 

 is absent. The body is rather narrow from side to side, and the 

 lower edge is usually sharp and toothed like a saw. This family 



