578 



FRESH-WATER BIOLOGY 



difficult than that of any other group of rotifers; it has been con- 

 fined mainly to specialists in this particular group. 



Many species of the Bdelloida possess 

 a remarkable power of withstanding dry- 

 ing. Philodina roseola is often found as 

 Httle pink balls in the dry deposits in 

 the bottoms of urns and eave-troughs. 

 When this material is placed in water, 

 the pink balls quickly swell, take the 

 rotifer form, and continue their inter- 

 rupted life activities where these were 

 stopped. Many species may be ob- 

 tained for study in the Uving condition 

 from dried moss and other vegetation 

 brought from a distance. No males are 

 known in the Bdelloida. 



26. The Seisonacea (Fig. 868) are ex- 

 traordinary rotifers parasitic on marine 

 Crustacea. Their relationships are un- 

 certain, but, having two ovaries, they 

 are usually placed near the Bdelloida. 

 In the Seisonacea male and female are 

 Since they are exclusively marine, these 

 forms are omitted from the synoptic key. 



The studies thus far made of the rotifers of different regions 

 seem to indicate that in general these animals may be said to be 

 potentially cosmopohtan, any given species occurring wherever the 

 conditions necessary to its existence occur. Whether any given 

 rotifer shall be found in a given body of water depends mainly, 

 not upon the locality of this body of water, but upon the precise 

 conditions there found. Studies on the rotifers of Europe, Asia, 

 Africa, America, and Australia show, not different faunas in these 

 regions, but the same common rotifers found everywhere, with 

 merely a new form here and there, and it is an extraordinary 

 fact that when a new rotifer is described from Africa or Australia, 

 its next occurrence is often recorded from Europe or America. In 

 stagnant swamps all over the world appear to be found the char- 



FlG. 868. Seison annulatus Claus, female, 

 (After Glaus.) 



similar and of equal size. 



