422 APPLIED BIOLOGY 



(4) The lung-fishes of South America, Africa, and Australia 

 deserve mention because they have gills and also primitive 

 lungs able to breathe air. This adapts them to life in places 

 where rivers are muddy or dry in certain seasons. The 

 American and African forms are true mud-fishes and bury in 

 mud at the beginning of the dry season. These fishes have 

 attracted much attention from zoologists because they sug- 

 gest a connection between fishes and amphibia. 



Important Groups of Fishes 



1. Cyclostomata sucker-mouth (hag-fish, lamprey). 



2. Elasmobranchii cartilaginous skeleton (sharks, rays, and 



skates). 



3. Teleostomi - bony skeleton { ganoids (sturgeon) 



[Teleosts (cod, perch, etc.). 



4. Dipnoi lung-fishes. 



348. Economic Value of Fishes. A few examples of 

 fishes which are important in the human food-supply will 

 suggest the enormous total value of this group of animals. 



Salmon worth more than $13,000,000 are annually caught 

 on the Pacific Coast of North America, nearly one-third of 

 these from Puget Sound and Columbia River. The average 

 weight of a full-grown salmon of the Columbia River species 

 is over twenty pounds, and individuals have weighed 100 

 pounds. 



Herrings are probably the most valuable food fishes in 

 the world. Huxley estimated that three billion, each aver- 

 aging half a pound in weight, are caught in the North Sea 

 and Atlantic annually ; and this is now too low an estimate. 

 They swim in enormous groups or " shoals " which some- 

 times extend over half a dozen square miles. " Sardines " 

 from Maine are simply small herrings, but the true European 

 sardines belong to another species. 



The codfish is one of the most important North American 

 fishes. About 7000 men are engaged in the fishery, and the 



