THE WATER FLEAS (CLADOCERA) 687 



found in the limnetic region and in the Great Lakes. Leptodora 

 is always limnetic in its habits. It is almost perfectly transparent; 

 the dark eye and yellow stomach alone being visible when the animal 

 is viewed by transmitted light. It is by far the largest of the 

 Cladocera, reaching a length of 18 mm. Its winter eggs hatch as 

 nauplii and this is the only species of Cladocera in which this 

 characteristic crustacean larva appears. 



The Cladocera have great economic value. Together with the 

 Copepoda they constitute the chief agency for converting the 

 smaller algae of fresh water into a form edible by the carnivorous 

 aquatic animals. They are the prey of insect larvae, which are in 

 turn an important item in the bill of fare of the larger fishes. Clado- 

 cera are themselves of great value as food for young fishes and there 

 is a period in the life of almost every fish when it feeds exclusively 

 on Entomostraca. Even the larger fishes do not disdain these 

 animals. The great spoonbill (Polyodon) fills its stomach with 

 Bosminae, or other tiny inhabitants of the water from which it 

 strains its food. 



The geographical distribution of Cladocera offers little of interest 

 that can be stated in a brief sketch, chiefly because the species are 

 so widely distributed. Some species, like Chydorus sphaericus, are 

 cosmopolitan. A majority of the species found in this country are 

 found also in Europe. Where a species is peculiar to this region 

 it is often but slightly different from the European form. The 

 student of Cladocera should presume that any species is probably 

 intercontinental, though it may prove to be more restricted in its 

 range. The study of our forms has not gone far enough to enable 

 us to speak of the local distribution of each species within the 

 general area which it covers, but it is known that the rare species 

 are very irregularly distributed. On the whole, the fauna of the 

 various regions of the country is strikingly similar, but with some 

 forms peculiar to each region. The southern states contain numer- 

 ous species which are common to them and to South America, 

 but are not found in the northern states. 



The student of Cladocera will find the cone net (p. 68) the best 

 agent for collecting the littoral forms. The catch should be put 

 into a cup, which should be filled with water, and the debris allowed 



