THE PROTOZOA 



225 



prepared in the laboratory, but only a few can be mentioned here. 

 Two rather common ciliated species are Vorticella and Stentor. 

 Vorticella is bell-shaped and attached to some object by a con- 



D 



FIG. 135. Types of protozoa. 



A, stentor; B, arcella ; C, difflugia ; D, globigerina, etc., in gray chalk; E, mas- 

 tigameba. (After various authors.) 



tractile stalk. Stentor (Fig. 135, A) is trumpet-shaped and may 

 be either attached or free-swimming. Some of the amebalike 

 species are protected by shells. The doughnut-shaped Arcella 

 (Fig. 135, B) and the pear-shaped Difflugia (Fig. 135, C) are 



