THE ANIMALS OF LAKES AND RIVERS 187 



growth of the more deMcate animals, and must be 

 a great bar to the passage of marine animals up estu- 

 aries and rivers. Almost any coast-line wiU illustrate 

 the same thing on a smaller scale. The shore collector 

 knows that to get the rarer and more dehcate animals 

 he must seek rocks or beaches remote from the mouths 

 of streams, for at the immediate outlets of these only 

 the handiest forms occur. This statement is not in- 

 consistent with that already made, that the waste of 

 the land is an important part of the food of the sea- 

 animals, for oxygen is a prime necessity which precedes 

 even the need of food, and few animals can take in 

 oxygen if their respiratory organs are choked with mud. 



But it is not only the suspended matter in the water 

 of lakes and rivers which is inimical to animal life. 

 The presence of large amounts of hme or manganese 

 salts, or of humic acid, may render the water unsuitable 

 to certain forms. 



StiU another point of great importance is the enor- 

 mous variation in temperature to which lakes and 

 rivers are subject. These variations are normally so 

 great that they render the water absolutely unsuited 

 to stenothermal animals. All fresh- water forms must 

 be eurythermal (cf. p. 160), and temperate and polar 

 forms must have special means of protecting them- 

 selves, or their offspring, against the cold of winter. 

 In the ' lakes ' of Victoria Land, which are almost per- 

 manently frozen, the naturahsts of Sir Ernest Shackle- 

 ton's expedition found rotifers which can apparently 

 tolerate being frozen into the ice for years, and will 

 yet hatch out if the temperature rises and the ice melts. 

 This is an extreme case, but it suggests the adaptations 

 necessary before animals can thrive in smaU masses of 

 water. Low temperatures are not, however, the only 



