THE WHEEL AmMALCUI.ES (ROTATORTA) 577 



which the animals attach themselves; it bears also a pair of ''spurs " 

 on its dorsal side, a short distance from the end. These spurs 

 perhaps represent the two toes of other rotifers. 



The trophi (Fig. 867) present perhaps the most modified type 

 found in the Rotifera; they show clearly that this group is not a 

 primitive one. In most species the trophi are represented by two 

 pieces shaped like a quarter of a sphere and placed side by side 

 (Fig. 867, A). Across the free surface of these pieces extend two or 

 more ridges. These jaws may be opened and closed by the mus- 

 cular mass in which they are imbedded, the ridges fitting together 

 in such a way as to serve as grinding teeth. The two halves of the 



Fig. 867. Jaws of Bdelloida. ^, Jaws of P/n'Wina Sryce/ Weber (typical ramate jaws). 

 Microdina paradoxa Murray. (After Murray.) 



B. Jaws of 



trophi represent the two rami of other rotifers, the remainder of 

 the apparatus having almost completely disappeared. But tran- 

 sitional forms (Fig. 867, B) show clearly how these trophi are de- 

 rived from the typical structure. 



The point in which the Bdelloida differ most from other rotifers 

 is in the fact that they have two ovaries in place of one. This 

 pecuharity is shared with the Bdelloida only by a bizarre group of 

 parasitic marine rotifers, the Seisonacea (Fig. 868) which live at- 

 tached to the marine crustacean Nehalia. On account of this pecu- 

 liarity the Bdelloida and Seisonacea are commonl}' classed apart 

 from all other rotifers as the Digononta, the others being called the 

 Monogononta. 



The Bdelloida include an immense number of species, the greater 

 part of them belonging to the genus Callidina. The difference 

 between species is often only slight, and the animals change form 

 almost continually, so that their systematic study is perhaps more 



