THE LIFE OF THE SIMPLEST ANIMALS 21 



emphasized. The shape of this one-celled body varies. 

 With the most primitive or simplest of the " simplest ani- 

 mals," like Amceba, for example, there is no " distinction 

 of ends, sides, or surfaces, such as we are familiar with in 

 in the higher animals. Anterior and posterior ends, right 

 and left sides, dorsal and ventral surfaces are terms which 

 have no meaning in reference to an Amoeba, for any part 

 of the animal may go first in locomotion, and when crawl- 

 ing the animal moves along on whatever part of its 

 surface happens to be in contact with foreign bodies." 

 The one shape most often seen among the Protozoa, or 

 most nearly fairly to be called the typical shape, is the 

 spherical or subspherical shape. Why this is so is readily 

 seen. Most of the Protozoa are aquatic and free swim- 

 ming. They live in a medium, the water, which supports 

 or presses on the body equally on all sides, and the body is 

 not forced to assume any particular form by the environ- 

 ment. The body rests suspended in the water with any 

 part of its surface uppermost or any part undermost. As 

 any part of the surface serves equally well in many of the 

 Protozoa for breathing or eating or excreting, it is obvious 

 that the spherical form is the simplest and most conven- 

 ient shape for such a body. It is interesting to note that 

 the spherical form is the common shape of the egg cell of 

 the higher animals. Each one of the higher, multicellular 

 animals begins life (as we shall find it explained in another 

 chapter of this book) as a single cell, the egg cell, and 

 these egg cells are usually spherical in shape. The full 

 significance of this we need not now attempt to under- 

 stand, but it is interesting to note that normally the whole 

 body of the simplest animals is a single spherical cell, and 

 that every one of the higher animals, however complex 

 it may become by growth and development, begins life as a 

 single spherical cell. 



12. The primitive but successful life. Living consists of 

 the performing of certain so-called life processes, such as 



