44 FRESH-WATER BIOLOGY 



important. It is a common characteristic of the eggs of rotiiiers, 

 of Crustacea, insects, and other arthropods, and also of the spores 

 and seeds of plants. Many crustaceans deposit eggs in the autumn 

 which require freezing before they will hatch. Some, as for exam- 

 ple those of the fairy shrimp (Eubranchipus) , require both summer 

 drying and winter freezing. The statoblasts of the fresh-water 

 Bryozoa germinate better after freezing or drying. Thus some 

 simple condition such as the rupture of the egg shell or covering 

 may be a requirement for growth as it is in some seeds. 



Any scheme that fails to consider the complete physiological 

 hfe history in relation to complete annual cycles is inadequate. 

 Still, because of the complexity of the problems involved simple 

 indices must be sought which will indicate the condition of waters 

 with reference to as many important factors as possible. These 

 indices must be selected with two facts in mind: First, that there 

 is in each annual cycle of the life of an individual or a species a 

 period of maximum sensitiveness; this falls at or near the breeding 

 period or at the time of appearance of young. Second, adequate 

 measure of hydrographic conditions are to be found in the peculiar 

 character of the annual rhythm rather than in the totals of this 

 or that factor for the year or a particular period. 



Many organisms, especially food fishes, deposit their eggs on the 

 bottom. It is to the bottom that the dead bodies of organisms 

 sink and at the bottom that they decompose and produce poi- 

 sonous substances in greatest quantity. Decomposition of the 

 bodies of plants and animals results finally in gases such as 

 ammonia, carbon dioxide, hydrogen sulphide, methane, etc. 

 The presence or absence of fishes and their animal food is con- 

 trolled by (a) their ability to recognize the presence of strange 

 or detrimental substances and to turn back when such are en- 

 countered, and (b) by their survival or death in situations where 

 they cannot escape the deleterious conditions. Their ability to 

 recognize common injurious substances has been shown to be 

 very marked and precise. The difference between different 

 species is one of degree and special habits. The effects of the 

 various decomposition products are the same in a wide range of 

 species with only slight differences in degree. The less sensitive 



