CHAPTER VII 



ANIMALS OF UNCERTAIN RELATIONSHIPS 



IN" this chapter we shall consider in a brief way a number 

 of different groups of animals whose relationships are un- 

 certain. Up to the present time the study of their habits, 

 structure, and development has been of too fragmentary 

 or unrelated a character to enable the majority of zoologists 

 to agree upon their classification. Nevertheless, many of 

 them are highly interesting and attractive, 

 often very common, and in some respects 

 they hold important positions in the animal 

 kingdom. 



69. The rotifers or wheel-animalcules. 

 The rotifers or wheel-animalcules are rela- 

 tively small and beautiful organisms, rarely 

 ever longer than a third of an inch, but at 

 times so abundant that they may impart a 

 reddish tinge to the water of the streams 

 and ponds in which they live. At first 

 sight they might be mistaken for one-celled 

 animals, but the presence of a digestive 

 tract and of reproductive elements soon dis- 

 pels such a belief. Examined under the 

 microscope, the more common forms are 

 seen to possess an elongated body terminat- 

 ing at the forward end in two disk-like expansions beset 

 along the edges with powerful cilia. These serve to drive 

 the animal about, or, when it remains temporarily attached 



FIG. 41. A wheel- 

 animalcule (Rotifer). 



