THE WHEEL ANIMALCULES (ROTATORIA) 579 



acteristic rotifers of stagnant water; in clear lake water are found 

 the characteristic limnetic rotifers; in sphagnum swamps every- 

 where, the sphagnum rotifers. Variation in the rotifer fauna of 

 different countries is probably due mainly to differences in the con- 

 ditions of existence in the waters of these countries, rather than to 

 any difficulty in passing from one country to another. The num- 

 ber of different sorts of Rotifera to be found in any given region 

 depends upon the variety of conditions to be found in the waters 

 of this region. Two bodies of water half a mile apart, presenting 

 entirely different conditions, are likely to vary more in their rotifer 

 fauna than two bodies of water 5000 miles apart that present 

 similar conditions. Of course, the tropics will have characteristic 

 species not found in cooler regions, since they present conditions of 

 existence not found elsewhere, and the same may be true of Arctic 

 regions. The problem of the distribution of the Rotifera is then 

 mainly a problem of the conditions of existence rather than of the 

 means of distribution. ' The ability of the eggs to live in dried 

 mud, which may be carried about on the feet of birds or blown 

 about as dust by the winds, seems to give sufficient opportunities 

 for any species to multiply wherever occur the conditions neces- 

 sary for existence. Most rotifers seem adapted to a rather nar- 

 rowly limited set of conditions. 



Many species of rotifers vary extremely in the external dftails 

 of their structure. This is particularly true of loricate rotifers 

 that bear teeth, spines, or other projections. Extreme examples 

 of such variation are seen in Brachionus bakeri Ehr. and in Anuraea 

 cochlearis Gosse (Fig. 913). Lauterborn shows that the variations 

 of Anuraea cochlearis are by no means haphazard, but depend upon 

 the seasons and upon changes in the conditions of existence. In 

 the course of a year this species undergoes a cycle of regular changes 

 from month to month, and this yearly cycle was found to be essen- 

 tially the same during a period of study of twelve years. In the 

 colder months of the year appear individuals of greater size, with 

 smooth loricas and long prominent spines. As the waters become 

 warmer, in spring and summer, the individuals found are smaller 

 in size, the surface of the lorica becomes roughened, and the spines 

 grow shorter, until the caudal one disappears completely. As cold 



