298 Descriptive Zoology. 



protozoans are composed of several or many cells, each of 

 which may have a pair of flagella. Thus there are three 

 principal modes of locomotion among protozoans : (i) slow 

 creeping by means of pseudopods, as in amoeba; (2) swim- 

 ming by cilia, as in paramecium ; (3) more active swimming 

 by flagella, this mode being not very unlike the second. 



The Shell-bearing Protozoans. Many protozoans have 

 shells. Some of these shells are composed of lime, others 

 of silica, while still others are formed of grains of sand 

 which the protozoan glues together by a secretion from its 

 protoplasm. Most of these shelled forms live in the ocean. 

 Some of the shells are borne where we should expect them, 



on the outside, the animal being 

 able to withdraw into the shell 

 and project again at will through 

 an opening. But in many the ani- 

 mal cannot thus withdraw itself 

 completely into the shell. Many 



of the shells are perforated by nu- 

 FIG. 173. NOCTILUCA. 



merous minute openings, through 



A phosphorescent marine protozoan. 



which fine threads of protoplasm 



are extended, these projecting threads sometimes forming 

 a network outside of the shell. In many forms the proto- 

 plasm increases in amount, flows out of the main opening 

 of the shell, and forms a new shell, larger than the old one, 

 but attached to it. In this way it proceeds, making a series 

 spirally arranged, similar in general appearance to a spirally 

 coiled snail shell, or the chambered nautilus. 



Chalk. One of the most abundant and best known of 

 geologic formations is made by protozoans. Chalk is 

 made up of the shells, such as above described, of a kind 

 of protozoan known as Globigerina. Myriads of these 

 protozoans live in the ocean. When they die, their skele- 



