80 THE SMALLEST LIVING THINGS 



one type with a small central (microspheric) chamber, the other 

 with a large central (macrospheric) chamber (see Fig. 44, page 

 77). The first type gives rise to asexually produced spores — the 

 second type gives rise to gametes which fuse two by two, thus 

 forming fertilized cells (zygotes) which develop into the micro- 

 spheric generation. The phrase alternation of generations is 

 used to describe this phenomenon — an asexual generation alter- 

 nating with a sexual generation. 



The Infusoria 



Characteristics 



In the infusoria* we find the most highly specialized types of 

 protozoa and the most complex forms of the single cell. Infu- 

 soria are found everywhere — in ponds, lakes, seas, in short, wher- 

 ever moisture is present. The derived organizations, while dis- 

 tinctly of one type, are highly varied, but each is always adapted 

 to its mode of life. Forms, movements, habits are always inter- 

 esting and frequently spectacular, making them a constant fasci- 

 nation to the microscopist. 



Cilia 



The most characteristic feature distinguishing infusoria from 

 all other protozoa is the possession of cilia. These are shorter 

 than flagella, are like an eyelash in form, and propel the organ- 

 ism through the water by the synchronous beating of many, after 

 the manner in which the oars of the Roman galleys were op- 

 erated. In one division of infusoria — the Suctoria — cilia are 

 present in the embryonic stages only, the derived organization of 

 the adult being characterized by the possession of suctorial ten- 

 tacles'*' through which food is taken into the body. In the other 

 group — the Ciliata (Ciliates) — cilia are present throughout life. 



The distribution of cilia on the body follows fairly definite 

 modifications of type and affords a means of classification of the 

 ciliates. Furthermore, fusion or coalescence of cilia gives rise to 

 motile organs of considerable complexity. In a generalized type 

 the cilia run in longitudinal rows from the mouth at one end to 



*The singular of "infusoria" is "infusorium," and the adjective is "infusorian." 

 t Elongated processes which are equipped for sucking. 



