266 
COMPARATIVE ZOOLOGY. 
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 
^ to "jpg* 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- 
( Hydatina ), highly periority to the Coral, which it imitates, 
magnified. j n possessing a distinct alimentary canal 
and a well-defined nervous system. The cells of a group 
never have connection with a common tube, as in Coelen- 
terates. There are both marine and fresh-water species. 
This group and the next following are related to the 
Mollusca. 
Class V.-—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 
