ONE-CELLED ANIMALS AND SPONGES. 119 
in projecting outward a portion of the body in the 
direction in which the animal wishes to move. The 
little swelling thus made is called a pseadopodiuni or 
false foot. Such false feet may appear at any time on 
any part of the body. When this pseudopodium has 
extended itself sufficiently the rest of the body seems 
to glide into it by a sort of flowing motion. The varying 
position and size of the pseudopodia give it its irregular 
outline when seen under the microscope. The Amoeba’s 
power of “contractility ” is sometimes spoken of as a 
separate activity corresponding to the contractility 
noticed in the muscles in higher animals. 
Thus this simple bit of protoplasm performs the same 
functions which are common to higher animals, all the 
activities in this case being carried on by a single cell. 
As we examine other animals we find it easy to arrange 
a series in which each animal is only a little more 
specialized than the one next below it, but such a series 
does not include all animals. It is interesting to note 
that such a series bears a strong resemblance to the 
various stages through which an egg (a single cell) of 
one of the more specialized members of the series 
passes in its growth. 
Rhizopoda with Shells. Some of the Amoeba-like 
animals cover their bodies with bits of mineral matter 
to form shells with openings 
through which the pseudopodia 
extend to gather food and as¬ 
sist in locomotion. In many 
cases these shells are secreted 
by the animal, that is, they are 
formed from the protoplasm 
of the body as our finger¬ 
nails are formed from our blood. Sometimes the 
pseudopodia resemble the roots of plants and hence 
these Protozoa which move by pseudopodia are called 
Rhizopoda. 
Some of the Rhizopoda secrete shells of calcium car- 
