SPONGES AND CORALS 



duty. Yet the flagella, as they whip the water and entice 

 it into the canals, are apparently no more than simple 

 appendages of the sponge itself, controlled by the sponge 

 as our arms and legs are controlled by us. But they are 

 really semi-independent individuals, and if we term them 

 ''devices", for want of a better name, we must be pre- 

 pared to regard them as living, self-acting ones. They 

 also occur in the Flagellata, motile unicellular organisms 

 present in numerous plants and bacteria ; in the sperma- 

 tozoa (minute active gametes in the semen of multicel- 

 lular creatures which serve to fertilize the ovum) ; and, 

 as cilia, covering the respiratory, excretory, and repro- 

 ductive systems of numerous animals, including our- 

 selves. 



Although some zoologists distinguish between flagella, 

 which occur singly or in very small numbers as ''whips" on 

 unicellular creatures, and cilia, which occur in large 

 numbers, the distinction cannot be justified. For the cilia 

 are never simple "hairs" like eyelashes (the name derives 

 from the Latin cilium, eyelids or eyelashes) but in all 

 respects — structure, size and activity — are similar to 

 flagella: both cilia and flagella are "whip-like" and 

 usually in incessant vibratile movement. 



In all such organelles — perhaps the best name for 

 them, for they are both organisms and small organs — 

 energy is somehow used to produce motion, but biologists 

 find it extremely difficult to explain the nature of the 

 process. One of our best authorities on the subject says 

 simply, "We do not know." Flagellated cells are found 

 in all the main divisions of the plant and animal king- 

 doms, with a few exceptions, and these cells serve either 

 of two purposes : to move the cells through the water, 

 when they are free-swimming creatures, or to move 

 water past the cells, when they are the organs of larger 

 organisms, like the sponge. 



Protozoa of all kinds live as parasites in the sense that, 

 even if they are not attached to larger creatures or (as 



149 



