248 



POLYZOA. 



The presence of decomposing animal matter in 

 water invariably congregates crowds of infusory ani- 

 malcules, which breed with astonishing rapidity? so 

 as to form a cloud of living atoms around the decaying 

 body, quite visible in the aggregate to the unassisted 

 eye. An animal thus seized, therefore, becomes a 

 centre to a crowd of infusoria, multitudes of which 

 must be constantly drawn into the tentacular vortex 

 and swallowed by the Polyzoon. 



^^>^, Besides the marine ge- 



nera above described, 

 there are many forms of 

 N; animals, belonging to this 

 ^''^^ class, that abound in our 

 fresh waters. These 

 Fluviatile Polyzoa are to 

 be met with in ponds and 

 streams, adherent to any 

 foreign bodies that may 

 i be casually submerged. 

 ^ Thus, they are found at- 

 tached to stones at the 

 bottom of the water, up- 

 on shells, upon leaves — 

 more especially those of 

 the water-lily and the bis- 

 tort — upon floating wood, 

 and upon the stems of 

 various plants. In order 

 to examine these beauti- 

 ful organisms in a living state, it is only necessary to 

 allow the leaf, or other substance to which they are 

 attached, to remain for some little time undisturbed 

 in a glass of clear water, when they will be soon seen 

 spreading forth their beautiful tentacula, as they pro- 

 trude from their delicate cells ; and by frequently 

 changing the water they may be kept alive for 

 months, affording objects of continual interest for 

 microscopical observation. 



When thus examined, it will be seen that the fresh- 



FlG. 187— PLITMATELLA. 



