144 



THE ANIMAL KINGDOM. 



Another remarkable group of animals belonging to the 

 crustaceans are those called Rotifer a, or wheel-bearers. 

 These are Infusoria that is, tiny animals which are in- 

 fused in water, and are so small that they can only be 

 seen by the aid of a microscope. The wheel-bearers are, 

 however, giants when compared with others amongst 

 which they live, for they are from one-fiftieth to one- 

 hundredth of an inch in size! When examined, it 

 appears as if a wheel were rapidly turning round, but this 

 is not so : the apparent revolving of the wheel is caused 

 by the bending and unbending of a fringe, consisting of 



Fig. 101. 

 ROTIFER, OR WHEEL-BEARING ANIMAL. 



a number of exceedingly delicate hairs called cilia. By 

 this motion a current is produced in the water, which thus 

 conveys the required -food into the open mouth of the 

 tiny creature. Fig. 101 shows a rotifer very highly mag- 

 nified. 



MOLLTJSCA. 



THE Mollusca are soft-bodied animals, having neither 

 internal nor external skeletons. Some arc merely covered 

 with a moist slimy skin, whilst others form a shell ; but 

 this shell is not any portion of the structure of the animal, 

 as it is in the Articulata, for we find that even in animals 

 which in other respects arc almost precisely alike, some, 



