COMPARATIVE ZOOLOGY. 
266 
muscles, where they become encysted. In burrowing, they 
cause great pain and fever, and sometimes death. The 
adult Worm is about ^ inch long. 
Class III.— Rotifera. 
The Wheel-animalcules, mostly found in fresh water, 
are minute Worms of few segments, having on the ante¬ 
rior end a disk ciliated on the edge, 
whence their name. They are from 
roi 7 to A of an inch long. They can 
bear drying and revivifying, like seeds. 
Class IY.— Polyzoa. 
These minute Worms resemble the 
Polyps in appearance, living in clusters, 
each individual inhabiting a delicate 
cell, or tube, and having a simple mouth 
surrounded with ciliated tentacles. The 
colony often takes a plant - like form; 
sometimes spreads, like fairy-chains or 
lace-work, over other bodies; or covers 
rocks and sea-weeds in patches with a 
delicate film. The majority secrete car- 
Fig. 219 . — Rotifer, or bonate of lime. A Polyzoan shows its su- 
( Hydaton ]™Wghiy periority to the Coral, which it imitates, 
magnified. j n possessing a distinct alimentary canal 
and a w T ell-defined nervous system. The cells of a group 
never have connection with a common tube, as in Ccelen- 
terates. There are both marine and fresh-water species. 
This group and the next following are related to the 
Mollusca. 
Class Y.— Brachiopoda. 
These Worms have a bivalve shell, the valves being 
applied to the dorsal and ventral sides of the body. The 
valves are unequal, the ventral being usually larger, and 
