300 PRINCIPLES OF ANIMAL BIOLOGY 



in suspension; hence light penetration is periodically very low. Tempera- 

 ture is nearly uniform at various locations in a stream at any one time, 

 but its variation seasonally for the stream as a whole is often extreme. 

 The oxygen content of s^viftly flowing water is generally near the satura- 

 tion point, and most of the characteristic brook animals cannot be rearee. 

 in a concentration of oxygen much below that level. 



Since the chief feature of streams, as distinguished from* lakes, is the 

 movement of their water, consideration of their animals will here be 

 limited to those whose currents are strong. This is the condition in 

 most small streams or brooks. In such streams, animals have to be able 

 to maintain their position; with the exception of the minute floaters, 

 they cannot as a rule allow themselves to be carried along by the current 

 and still be successful. One method of holding their places is to be 

 attached to fixed objects. That is a feasible method in general, since 

 animals do not have to travel in search of food, for it comes to them. 

 In very swift water, one finds the larvae of the black fly, Avhich hold fast 

 by adhesive organs at the posterior end, while their appendages are so 

 construct;gd as to strain minute organisms out of the water that flows 

 through them. Some of the caddis fly larvae spin nets on stones or other 

 objects in rapids; they cling to the net, which also serves to catch food as 

 the water goes through it. The other most abundant insects in brooks 

 appear to be the larvae of midges, which live on or in the bottom, and May 

 flies and stone flies of clinging varieties. In other animal groups there 

 are snails, flatworms (planarians), amphipod Crustacea, and mites. 

 Some algae form incrustations on rocks and other objects. In the same 

 brooks, but in the quieter water, are miller's-thumbs (fish) lurking under 

 overhanging banks, catfishes which lie close to the bottom, and darters 

 (fish) which are strong sudmmers. The larger rivers contain larger 

 animals, but they differ less from lake inhabitants. 



The rate of reproduction of brook dwellers must be high, since the 

 risks of loss are large. An individual that loses its station, if dependent 

 on attachment, is not likely to become reattached until it reaches slow- 

 water, and there the conditions are not usually favorable. A single pair 

 of midges, producing four generations a year, have a potentiality of 

 nearly eight million descendants, but on the average only two are pro- 

 duced and live, in each generation, to do what their progenitors did. 



Marine Conditions. — The oceans are so huge and are subject to so 

 many variable influences in their various parts that no .simple descrip- 

 tion of their environmental organization is possible. Their waters are a 

 little heavier because of the salt content, hence offer more support to 

 animal bodies than fresh water does. Deep water is at very high pres- 

 sure, but this feature is of little significance to any animals except those 

 fishes which have a closed swim bladder which is compressible. \'ery 



