70 EARTHWORMS AND THEIR ALLIES [ch. 



worm whose specific name has been mentioned. But 

 it is possible that others are similarly affected. And 

 it is highly important to note the prevalent habit 

 among the Tubificidae of lying with the head end 

 imbedded in the mud of the pool which they inhabit, 

 while the tail end emerges and waves freely in the 

 flood. The additional fact that this tail end occasion- 

 ally bears gills (as in Branchiura soiverbii and 

 Phreodrilus branchiatus) has a collateral importance 

 not to be mistaken. 



CHAPTER V 



RELATIVE FREQUENCY OF EARTHWORMS IN 

 DIFFERENT REGIONS OF THE WORLD 



It will be of use for various purposes to be con- 

 sidered later to arrive at a comprehensive view of 

 the relative numbers of species and genera of earth- 

 worms in the four quarters of the globe. And in 

 making this general census we shall not take into 

 consideration the purely aquatic forms, but shall 

 limit ourselves to the earthworms, sensu stricto, or 

 Megadrili, of which, however, it is true that some 

 members are actually lake and river dwellers. This 

 latter fact will not, however, interfere with the 

 usefulness of the comparative survey. 



