8o FRESH-WATER ANIMALS 



much about, but the full extent of which we too 

 often fail to realise. Very large numbers of every 

 species of animal perish before reaching maturity. 

 If we reflect for a moment on the really enormous 

 number of eggs which fish and other marine animals 

 lay, it is clear that if all these eggs, in the case 

 of any single species, were to come to maturity, 

 there would very soon be little room in the sea for 

 anything else. A mackerel of a pound weight, for 

 instance, will lay eighty thousand or ninety thousand 

 eggs, a cod may produce five millions, and a conger- 

 eel no less than fifteen millions. However, the vast 

 majority in every species never come to maturity, 

 but are devoured while young by other animals, as 

 food ; while a very small minority, favoured by the 

 possession of some slight accidental advantage, 

 escape destruction, survive, and perpetuate the 

 species. So keen is the struggle for existence in 

 the sea, especially in the shallow waters round the 

 coasts, where life is far more abundant and 

 competition more severe than at greater depths, 

 that many forms, belonging to different groups, 

 have, to escape it, worked their way up the rivers, 

 and adapted themselves to fresh-water life. 



Let us now see what these fresh-water animals 

 are like, to what groups they belong, and what are 

 their most important characteristics. The first of the 

 eight large groups into which the animal kingdom 

 is usually divided is that of the Protozoa. These 

 include the simplest unicellular forms of animal 

 life, and occur very abundantly in fresh water, 

 though certain important subdivisions e.g., the 



