310 



ANIMALS IN INLAND WATERS 



merit. Thus Plumatella princeps var. spangiosa overruns snail shells 

 until they resemble small potatoes. 14 Fresh-water sponges (Ephydatia 

 fluvialitis) attach themselves in like manner. Other invertebrates are 

 somewhat less abundant than the detritus feeders.* 



The large development of lower animal life on the nutritive ele- 

 ments of the mud and detritus which thickly cover the bottoms of 

 slowly flowing streams affords the basic food supply for the large 

 numbers of fish which are found in the lower streams. These are pri- 

 marily bottom feeders such as carp, catfishes, and suckers, together 



Mi 



Fig. 85. — a, Piece of clay from the bank of the Marne, below the water 

 level, riddled with the burrows of may-fly larvae {Palingenia) ; some with 

 double, others with single openings, b, Longitudinal section through one of these 

 burrows. After de Reaumur. 



with sturgeons and spoonbills, the African Mormyridae, and many 

 others; predaceous fish in turn are associated with the herbivores and 

 detritus feeders. Experience in fishing shows that, the farther a fisher- 

 man lives from the mouth of a river, the greater the area of water he 

 must have, and while a lake with a yearly net profit of 30 marks per 

 hectare is extraordinarily good, there are places in the lower Oder 

 where the profit is 100 marks, and in the lower Elbe even 160 marks 

 net profit may be realized in a hectare. 



The number of migratory fish is also, of course, largest in the 

 lower reaches of the river. Many stop here, and those that go farther 

 upstream must also pass through the lower river. These include prin- 

 cipally species coming up to spawn, e.g., Salmonidae and sturgeons. 

 The flounder {Pleuronectes platessa) comes up into European rivers 

 only in search of food, but it never becomes sexually mature in fresh 



* Among snails, the gill-breathing forms such as Vivipara fasciata, Neritina, 

 Valvata, and Bythinia are rather plentiful. There are the flatworm Dendrocoelum 

 lacteum, the leech Herpobdella, the water isopod Ascllus, and others. 



