228 MICROSCOPY FOR BEGINNERS. 



mals seldom escape unless the captor is willing. And 

 not only are the tentacles used to capture the food, but 

 " for a multitude of other offices. They are each capa- 

 ble of independent motion, and may be twisted or 

 turned in any direction ; bending inward, they take up 

 and discard objectionable matter, or push down into the 

 stomach and clear the oesophagus of food too small to be 

 acted on by the parietal muscles." 



To examine the Polyzoa under the microscope de- 

 mands a very deep cell to hold a large quantity of water 

 and to prevent the cover-glass from pressing on the 

 bodies. It is often better to place the microscope in an 

 upright position and omit the thin cover. In this ar- 

 rangement the water trembles easily, and not only inter- 

 feres with the distinctness of the image, but terrifies the 

 timid creatures on the slide. Thte observer must, there- 

 fore, be careful not to touch the table, and to make his 

 examination in a quiet room. They will ask a little at- 

 tention and some gentle treatment, but what they will 

 show with the help of a one-inch objective will amply 

 repay the outlay of time and patience. 



The following Key to the genera will help the stu- 

 dent to name the forms he may find. 



Key to Genera of Fresh-water Polyzoa. 



1. In a jelly mass (a). 



2. In adherent, branching cylindrical tubes (J). 



3. In adherent, branching colonies formed of tubular, 



club-shaped cells (G). 



