6 THE PROTOZOA THE DAWN OF LIFE 



but a single protoplasmic cell, yet it is capable of all the functions 

 of a multicellular animal movement, feeding, growth, reproduc- 

 tion and that in it we have, to quote Professor Haeckel, " an 

 approximate illustration of the ancient common unicellular an- 

 cestor of all the Metazoa, or multicellular animals." 



Further, a simple Amoeba has a striking resemblance to the 

 " primary cell " or " ovum " of all animals, whether vertebrate 

 or invertebrate. It may be regarded as equivalent to this uni- 

 cellular stage which is the beginning of all the higher organisms. 

 The so-called "white corpuscles" or leucocytes of our blood are 

 amoeboid. As they circulate along the blood-vessels, they execute 

 movements like those of Amoebae, constantly modifying their 

 shape, and engulfing foreign substances, or organisms such as 

 bacteria, which have entered the system, just in the same manner 

 as the Amoeba takes in its food. 



While the distinction between the higher plants and animals 

 is perfectly sharp and obvious, it becomes increasingly difficult 

 to maintain such a distinction as the two groups are traced down- 

 wards to ever simpler forms, until they merge in an assemblage 

 of organisms which partake of the characteristics of both king- 

 doms. Of the Protozoa certain groups are distinctly animal in 

 their chemical activities or metabolism, in their mode of nutri- 

 tion, and their locomotive powers ; while in others it is very 

 difficult to draw a distinction between them and the lower fungi, 

 making it extremely difficult to separate sharply the Protozoa 

 from the Protophyta, the lowest animals from the lowest plants, 

 and therefore the term Protista, first introduced by Professor 

 Haeckel, is now generally applied to designate these lowly forms 

 of life, organisms which do not consist of an aggregation of 

 differentiated cells. 



A large number of the Protozoa resemble Amoeba in the pos- 

 session of pseudopodia, or false-feet, and these pseudopodia-bearing 

 organisms constitute one of the great divisions or classes into 

 which the Protozoa are divided the class Rhizopoda. 1 But it 

 is only a comparatively small proportion of these Rhizopods 

 which, like Amoeba, possess comparatively short, blunt pseudo- 

 podia, and they are consequently grouped together by this 

 natural distinguishing feature to form one of the leading divisions 



1 Greek, rhixa, a root ; pous, a foot. 



