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- 
trodinchnank pels such a belief. Examined under the 
animaleule (Rotifer. 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 
66 
