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 

 srinr to -sir of an inch long. They can 

 bear drying and revivifying, like seeds. 



CLASS IV. 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- 

 FIO. 219. Rotifer, or bonate of lime. A Polyzoan shows its su- 



" Wheel-animalcule " . , ~ ,,.,... 



(Hydatina), highly periority to the Coral, which it imitates, 



in 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 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 



