THE CELL 35 



"plant animal" which the botanist puts under the plant division 

 Flagellata (plants provided with flagellae for swimming), while 

 the zoologist regards it as a Protozoan (or "primitive animal"). 

 The fact that Euglena contains chlorophyll, the green pigment 

 of plants, suggests a plant origin, but the fact that it swims indi- 

 cates animal characteristics. The botanist accepts this latter 

 deduction and leaves Euglena for the zoologist. But it makes 

 little difference. On the contrary, it is to be expected that 

 border-line organisms exist. If plants and animals arose from 

 a common ancestor, it is only natural that primitive forms neither 

 distmctly plant nor distinctly animal should remain. 



Fig. 24. — Amoeba proteus. {From J. Leidy.) 



All of the many kinds of one-celled plants and animals are 

 classed together as Protista. Formerly they were referred to, 

 but now only in a popular sense, as "animalculae." The ani- 

 mals among them are known as Protozoa. The most renowned 

 of the Protozoa is Amoeba (Fig. 24). This unicellular animal 

 has become well known to the layman in such expressions as 

 "from amoeba to man," which suggests that Amoeba is the 

 most primitive of living beings. As generally illustrated, 

 Amoeba is shown as a lobed body with a spherical nucleus, a 

 large contractile vacuole, several food vacuoles, and many 

 protoplasmic granules. S. 0. Mast has observed far greater 

 details. His drawing of Amoeba proteus is so clear that no fur- 

 ther description is needed (Fig. 25). 



Some indication of how multifarious are the varieties of 

 Protista, or one-celled plants and animals, is to be had from a 

 mere listing of a few of them. A familiar form is the green alga 



