8 THE PROTOZOA. 



2. The endosarc contains no food-vaouoles, owing to the 

 fluid nature of the food ; this also accounts for the 

 absence of a mouth, the nutriment being absorbed 

 by the whole surface of the cell-body. 

 a. The nuclei are numerous. 



C. Reproduction. 



This is effected by oblique or transverse fission. 



Kill the OpalincB by running a drop of acetic acid (1 per 

 cent.) under the cover-glass ; and then stain with safranin or 

 carmifie. 



The nuclei are well seen in specimens so treated. 



D. Encystment. 



Opalins have been observed to become encysted, or en- 

 closed in a cyst, and in this state they leave the body of the 

 frog and are subsequently swallowed by tadpoles. 'Within the 

 cyst the Opalina divides into numerous small spores, which 

 are liberated in the tadpole and develop into Opalinse. 



IV. VORTICKLLA. 



Vorticella, which is just recognisable with the naked eye, 

 differs from the two preceding infusorians in being attached 

 by a contractile stalk to plants or other objects, often to other 

 aquatic animals ; it may, however, under certain conditions, 

 become detached from its stalk and swim freely. 



Both freshwater and marine forms of Vorticella are known. 

 The following description applies more particularly to some 

 of the larger freshwater species, but most of the marine forms 

 closely resemble these. 



Vorticella may be distinguished from allied genera which 

 are often found with it by its unbranched stalk, and by the 

 spiral form which this assumes on contraction. 



Mount a specimen in a drop of luater together with a hit 

 of the weed or other substance to which it is attached. 

 Examine it microscopically. 



