PROTOZOA 153 



4. Reproduction is effected chiefly by dividing into two or 

 more parts or cells, which occasionally remain associated. The 

 nucleus, when present, divides with the division of the cell 

 (Fig. 7). 



FIG. 68. 



t.c 

 t.n. 



FIG. 68. Paramecium in optical section (semi-diagrammatic). A, anterior end; c, cilia; t.c., 

 ectosarc; e.n., endosarc; /.., food "vacuole"; g, gullet; N, meganucleus; n, micronucleus; o, oral 

 groove, leading to the mouth; p.v., pulsating vacuoles in different stages of contraction; tr., tricho- 

 cysts; v, food vacuole in process of formation. 



Questions on the figure. In what sense is the term "vacuole" descriptive of 

 the structures to which it is applied in Paramecium? Describe the special adapta- 

 tions of the anterior end. Judging from their distribution have the cilia any other 

 function than locomotion? In what way are the food vacuoles formed? Why do 

 some food vacuoles appear lighter than others? 



194. Habitat. Protozoa in their active stages require 

 abundant moisture, hence they are found in water, fresh or 

 salt, and as parasites in the bodies of other, animals. The 

 Sporozoa are parasitic. Some amoeboid Rhizopods infest the 

 digestive tract of man and other animals, producing irritation 

 and disease. The Infusoria occur in water in which there is 

 decaying organic matter and minute organisms of various 

 kinds. Vohox and Euglena, green forms often classed as 

 Protozoa, have the power which green plants possess of using 

 the inorganic substances found in ordinary water in building 

 up their substance. 



195. Organization. We cannot say that Protozoa have 

 organs in the sense in which we have defined that term hitherto, 

 yet they are certainly organized. The organization shows 

 itself in the nucleus, in the distinction of ectosarc and endosarc, 



