tion is external in most egg-laying forms. If any nest is built or care exercised 

 over the young, it is usually the male that assumes most of the responsibility. 

 Many of the marine fishes lay eggs of the floating or pelagic type. These, being 

 exposed to many dangers, are produced in immense numbers. It is estimated 

 that the average adult codfish produces about nine million eggs at one time. 

 Most of the fresh'V/ater fishes lay eggs of the sinking or demersal type. A fif' 

 teen pound carp lays about two million eggs, which are allowed to fall among 

 vegetation, to which they adhere. A salmon of about the same si2;e buries its 

 crop of about 17,000 eggs in the gravel of a stream, where they will be fairly 

 secure from predators and yet be well aerated. Some fishes, such as the sun- 

 fishes, clear out circular depressions on the floor of the lake and stand guard 

 over the eggs as they develop. The male will bite vigorously at anything that 

 intrudes, but it is a poor sportsman indeed who will profit by this habit to get 

 a string of fish. A few moments spent in watching the nest will convince the 

 observer that minnows and snails make the job of guard no easy one and that 

 the batch of eggs would have little chance of survival if the gallant defender 

 were removed. The common sunfish produces about 5,000 eggs. Catfishes 

 generally lay their eggs in a hollow sunken log or in a hole in the bank. The 

 male watches over the eggs and later takes the school of young out on excur' 

 sions as they begin to travel. A few fishes, such as the sticklebacks, build 

 elaborate nests, much like those of birds, among the water weeds. In general, 

 greater parental care in selecting the nesting site or caring for the young com- 

 pensates for a reduction in the number of eggs, the end result being that each 

 group maintains its numbers in the biotic equilibrium. 



Fishes afford good material for the study of biological problems. A few 

 fields of investigation in which our knowledge is still incomplete are suggested 

 here. 

 Weberian apparatus — comparative studies of its development in different 



species; tests on its functions. 

 Air bladder — exchange of gases; gas content under various environmental 



conditions. 

 Migration — tagging experiments on (a) random wandering and its relation to 



natural restocking of areas depleted by temporary pollution or overfishing; 



(b) directed movements — migration for reproductive purposes or with 



seasonal changes. 

 Distribution — limitation of ranges through natural barriers and through fail- 

 ure in adaptation. Recent changes due to intentional introduction or to 



accidental introduction as bait or by canals and floods. 

 Species — limits of species, relation to subspecies. Ecological varieties. Natural 



hybrids. 

 Growth — determination by scales and otoliths. Correlation of growth rate 



with environmental conditions — food supply, pollution, limitation of 



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