PROTOZOA TO WORMS 39 



leading an independent life. There are special forms 

 of cells besides those described ; nettle cells for capt- 

 uring food, interstitial cells, etc., bub these do not con- 

 cern us. 



' The distance from the single-celled amoeba to hydra 

 is vast, probably really greater than that between any 

 other successive terms of our series. It may therefore 

 be useful to consider one or two intermediate forms 

 and the parallel embryonic stages of higher animals, 

 and to see how the higher many-celled animal origi- 

 nates from the unicellular stage. 



The amoeba is an illustration of a great kingdom of 

 similar, practically unicellular forms, which have played 

 no unimportant part in the geological history of the 

 globe. These are the protozoa. They include, first of 

 all, the foraminifera, which usually have shells com- 

 posed of carbonate of lime. These shells, settling to 

 the bottom of the ocean, have accumulated in vast beds, 

 and when compacted and raised above the surface, 

 form chalk, limestone, or marble, according to the de- 

 gree and mode of their hardening. 



The protozoa include also the flagellata, a great, 

 very poorly defined mass of forms occupying the 

 boundary between the plant and animal kingdoms. 

 They are usually unicellular, and their protoplasm is 

 surroimded by a thin, structureless membrane. This 

 prevents their putting out pseudopodia as organs of 

 motion. Instead of these they have at one end of the 

 ovoid or pear-shaped body a long, whiplash-like pro- 

 cess or thread, a flagellum, and by swinging this they 

 propel themselves through the water. These flagel- 

 lata seem to have a rather marked tendency to form 

 colonies. The first individual gives rise to others by 



