144 



ZOOLOGY. 



believed to be generative organs, the nucleus being an 

 ovary, and the nucleolus a mass of sperm cells. It has, 

 therefore, been concluded that these animals are herma- 

 phrodite, reproducing their kind by fertile ova. 



210. Another mode of reproduction is termed fission 

 (Lat. fissus, a cleft), or self-division. The body of a Par- 

 amcecium splits into two parts, each of which becomes an 

 independent being. A well fed animal of this group has been 

 observed to divide in this manner every twenty-four hours. 



211. The Vorticella (diminutive of Lat. vortex, a 

 whirlpool), or bell-animalcule, may be taken as the type 

 of the fixed forms of Infusoria. It may be found attached 

 to the stems of the duck-weed, and other aquatic plants. 

 To the naked eye a group of these animals gives a musty 

 appearance to the stem of the plant on which they are 

 located. When viewed by a microscope they appear as 



beautiful little bells or 

 vases, attached to stalks 

 which are eight or nine 

 times their own length. 

 This stalk is a tube, and 

 the animal has the power, 

 when alarmed, of contract- 

 ing it into a spiral form 

 by means of a filament 

 which passes through it 

 in a longitudinal direction. 

 At the margin of the 

 bell there is a projecting 

 rim which surrounds a 

 circular space called the 

 FIG. 59.-VORTICELLA. di sc< This disc is separ- 



ated from the rim by a groove, and is furnished at its 

 outer margin by a fringe of cilia, forming a spiral line 

 which is carried one or more times round the circumfer- 

 ence. The animal has the power of retracting the diso 

 with its ciliated fringe into the interior of its body. 

 The layers of the body are similar to those found in 



