WHEEL-ANIMALCULES. 45 
agquaticus is the thread-worm, common in many insects 
and in pools. They are the so-called horse-hairs that do 
not turn to snakes. 
Class III_— WHEEL-ANIMALCULES (Rotifers). 
The Rotifers (Fig. 44) are microscopic worms, some 
being only zs of an inch in length, having a membranous 
covering. Upon the anterior por- 
tion they have one or two disks, 
surrounded by cilia (Fig. 44, A), 
whose constant motion creates 
the optical illusion of wheels re- 
volving. They abound in the 
ocean, and in standing fresh wa- 
ter, and are so tenacious of life 
that they recover after having 
been dried for years, their sud- 
den appearance after continued 
droughts being attributed to what 
is called spontaneous generation. 
Under the microscope they as- 
sume remarkable shapes: now 
oval, caused by the upper and 
lower segments shutting togeth- 
er; then leech-like, moving along 
seemingly by suckers, or darting 
off swiftly, propelled by cilia. Fic. 44.—A Rotifer, highly 
They are oviparous, and Ehren- magnified (Hydatina sen- 
. ta). A, cilia; a, anus; 
berg found that one species pro- b, contraniily vedele ; 
duced 16,000,000 young in twelve’ water-vessels ; ¢, Ovary ; 
days. J; ganglion. 
Class IV.—Moss ANIMALS (Polyzoa). 
These animals form a moss or coral-like growth, com- 
posed of cells (Fig. 45), each of which contains a minute, 
