THE PHYLUM PROTOZOA 



157 



small food particles brushing against the collar stick tightly and are 

 carried to the collar base. Periodically, at the base, small pseudopods 

 erupt and engulf the food in a food vacuole, where it is digested. This 

 group includes a variety of colonial forms. Individuals tend to secrete 

 a gelatinous material around their bases, which in colonial species may 

 form a bulky structure. The more complex colonies (Fig. 8.7) include 

 ones with branching patterns and one free-swimming spherical species, 

 Sphaeroeca volvox. 



These are but a few of the flagellate groups. Many flagellates are 

 parasitic; the most important of these are the trypanosomes that pro- 

 duce sleeping sickness. These and other parasitic protozoa will be dis- 

 cussed in Chapter 39. 



49. Class Sarcodina 



Unlike other protozoa, the Sarcodina have no definite body shape. 

 Because of their ameboid movement the shape changes from moment to 

 moment. Nonetheless shape can be helpful in identifying species. Some 

 species of amebas, for example, form several long narrow pseudopods 

 at one time, while others form only one or two, or the pseudopods may 

 be short and blunt (Fig. 8.8). It is possible to describe an "average 

 shape" for a given species. The internal structures occuj^y no particular 

 position. Nucleus, contractile vacuole and food vacuoles shift about as 

 the animal moves. 



Amebas. Amebas (order Amoebozoa) are common in all waters, 

 move slowly, and are easily studied under the microscope. The ecto- 

 plasm (cytoplasm near the cell surface) is clear, while the endoplasm 

 or inner cytoplasm is granular. From the behavior of the granules, it is 

 apparent that the outer part of the endoplasm is in the gel state while 



Figure 8.8. An ameba. The animal is flowing to the right. 



