558 FRESH-WATER BIOLOGY 



may be floating in the water, or that are easily washed from sur- 

 rounding objects. The mouth, situated in the posterior part of 

 the corona, opens, and so admits or seizes such food as is adapted 

 to the rotifer. In many rotifers the cilia are the chief direct agents 

 in obtaining food, and in practically all species they are either 

 directly or indirectly of the greatest importance for this function. 



FIG. 859. Corona of Proales tigridia Gosse. A, surface view, from ventral side. B, side view. 

 m, mouth. (After Wesenberg-Lund.) 



3. In place of bringing food and oxygen backward to the rotifer, 

 the cilia may carry the animal forward to new supplies of these 

 necessities. This is the case in all free-swimming rotifers; the cilia 

 are the main organs of locomotion. In thus moving the animals 

 about, the cilia of course play as important a part in food-getting 

 as when they bring the food to the rotifer. In most species the 

 cilia act in both ways at once, bearing the animal forward and the 

 food backward, so that the two meet. 



4. The water currents remove the products of respiration and 

 excretion, which the rotifer, like other animals, is continually 

 giving off. Carbon dioxide is doubtless given off over the whole 

 surface of the body, while other waste products are discharged by 

 the contractile vesicle (see p. 561). If these waste products were 

 allowed to accumulate, they would be most injurious. 



While these are the main uses of the cilia, they assist, in a num- 

 ber of rotifers, in other important operations, such as the con- 

 struction of a tube or nest. 



The further course of the food may now be followed. The mouth, 

 situated in the posterior part of the corona (Fig. 859, m), leads into 

 a cavity with thick, muscular walls, known as the mastax (Figs. 856 

 and 857, mx) . The mastax is armed with a complicated set of jaws, 

 which have little resemblance to jaws found anywhere else in the 

 animal kingdom. They are known as the trophi (Fig. 857, A, tr). 



