MISCELLANEOUS MINOR PHYLA 



165 



Dorsa! view 



Lateral view 



Figure 87. Phylum Rotifera. General structure of a female rotifer. Left, dorsal view. Right, 

 lateral view of the same animal. 



contents into the cloaca and then out the 

 cloacal opening. This bladder, besides being 

 excretory in function, doubtless serves to 

 maintain a proper water balance in the ani- 

 mal, just as the contractile vacuole does in 

 the protozoans. 



The sexes of rotifers are separate. The 

 males are known in only a few species; and 

 where found, they are usually smaller than 

 the females, and often degenerate. Two 

 kinds of eggs are laid: the summer eggs, 

 which develop parthenogenetically, are 

 thin-shelled and commonly of 2 sizes; the 

 >arger produce females and the smaller pro- 



duce males. The winter eggs, which are fer- 

 tilized, develop into females and have thick 

 shells which protect the contents during un- 

 favorable weather. 



One peculiarity of rotifers is their power 

 to resist desiccation. Certain species, if 

 dried slowly, secrete gelatinous envelopes 

 which prevent further drying; in this condi- 

 tion they live through seasons of drought 

 and may be subjected to extremes of tem- 

 perature without dying. 



The resemblances between certain rotifers 

 and the trochophore larvae of certain mol- 

 lusks, annelids, and other animals to be 



