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. — Botifera. 

 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 

 jurr to ^V 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- 

 FiQ. 219. — Eoiifer, or bouatc of Hmc. A Polyzoan shows its su- 



" Wheel-auimalcule " • •. . . r-i i i ■ i • . . 



{ Hydatiiw, ), highly periority to tlic (Joral, 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 Coelen- 



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 



