CELLS AND CILIA 15 



flinty shells are commonest in the deeper parts of the ocean, 

 which in some cases is five miles below the surface of the 

 water. It is quite impossible to put into any kind of fif^ures 

 that can be realized the incredible numbers of these minute 

 organisms which have built up and are building up so large a 

 portion of the earth's crust ; even a Russian bank-manager used 

 to the Soviet currency would hardly grasp their significance. 

 As a rule, animals with one cell are not visible to the naked 

 eye, but there are certain unicellular animals that are parasitic 

 in the bodies of higher animals which are clearly to be seen 

 without magnification. A parasite is an animal that lives in 

 another plant or animal — called the "host" — and obtains 

 its nutriment at the expense of the host. Certain unicellular 

 parasitic animals called Gregarines, which live in earthworms, 

 are quite visible to the naked eye, and one which lives in the 

 lobster attains a length of tw^o-thirds of an inch. 



Cilia and Flagella 



Amoebae move by a creeping or gliding action, but many 

 unicellular plants or animals are propelled or rowed along 

 by vibrating processes called flagella. When a cell has a 

 number of flagella all vibrating in unison, they are called 

 cilia. Cilia play so large a part in life, not only in Protozoa 

 but in most of the higher animals, that they deserve a special 

 section of this chapter. 

 Cilia are like eye-lashes 

 standing out from the 

 body of a cell. There 

 may be only one, as is 

 the case with many 

 unicellular plants or -pm. 4. Eughna viridis, which swims by the 

 animals or the male lashing of a single flagellum at the front end. 

 1 . • n„ ^f X 100 1. Mouth. 2. Contractile vacuole. 



reproductive^ cells of 3^/^l^g J,t spot or eye. 4. Nucleus, 

 plants or ammals, the 



anthcrozoids and the spermatozoa, and then, as we have said, it 

 is called a flagellum. Such a flagellum may be at the front end 

 or the hind end of the cell. In the former case it laslics about, 

 sometimes in a spiral, and draws the cell after it. Or it may 



