234 THE AMEEICAN MONTHLY [October, 



gray squirrel as such or as a sciuroid, rodent, mammal, vertebrate 

 animal or even living thing. We are thinking of different pecu- 

 liarities in each case as we use the various names of the squirrel 

 as representing those characters. 



The fact that the correlated groups are serial among themselves 

 and also stand in relation to other series of groups is an easily 

 demonstrated fact and will appear at once as we examine the pro- 

 tozoa. The present article will be an attempt in the limited space 

 available for the purpose to set forth the classes and orders of phy- 

 lum and to examine it biologically for the purpose of discovering, 

 if possible, the serial arrangement of its members and an explana- 

 tion of the serial arrangement in biological terms. 



AmcEba proteus (see figure i). This animal has been de- 

 scribed so often that even a summary of its characters appears to 

 be trite. It can easily be found on decaying submerged vegeta- 

 tion. It is a mass of protoplasm with nucleus and contractile 

 vacuole and several masses of food enveloped by the protoplasm. 

 It is sensitive to temperature and other forms of energy applied as 

 stimuli. It has the power of thrusting out its substance in the 

 form of transitory pseudopodia, which may arise at several differ- 

 ent places at the same time. These may enlarge, the remainder 

 flowing into one of them and the body thus move forward. 

 Amceba proteus is not the simplest conceivable term in the series 

 of animal and plant life. The contractile vacuole and the nuclei 

 are both specialized portions of the protoplasm, and a still sim- 

 pler condition would be presented by a cell in which the former 

 was wanting and the latter was not differentiated from the other 

 protoplasm. The pseudopodia are formed first in the outer clear 

 portion or ectosarc, and this portion of the substance may not im- 

 probably have motor and nervous functions, while the centre has 

 separate anabolic functions by means of which it metabolizes food 

 to more protoplasm. The nucleus remains permanently in the 

 centre of the body notwithstanding the many changes of outline, 

 and the contractile vacuole is always posterior or opposite the end 

 toward which the animal is moving. Though the contractile 

 vacuole, nucleus, and ectosarc must be considered to be differenti- 

 ated parts of the body, A?nceba frotetis must still be regarded as 

 a very simple animal in comparison with others of the phylum, 

 and yet it is one from the like of which the others can be con- 

 ceived to have originated. 



Amoeba radiosa (Fig. 2) is found less frequently than 

 A. proteus but can often be seen in similar situations. It has a 

 body with nucleus and contractile vacuoles, but its pseudopodia 

 are long and tapering and remain swaying back and forth for a 

 long time with a slow sweeping motion. They can be drawn in 

 and the body become spherical in outline, and hence are not per- 

 manent like flagella and cilia in the Class Infusoria. This form 

 of pseudopodia, which can be described as filamentous motile 

 prolongations of the ectosarc, differ from the permanent organs 



