26 PRIMITIVE ANIMALS 



and the contrary doctrine that all life proceeds from 

 pre-existing life is universally accepted, the origin 

 of life is necessarily shifted back to a remote and 

 intangible past when the conditions of existence 

 were unimaginably different from those at the present 

 time. 



It is very difficult to believe that life originated 

 in anything like the form presented to us to-day by 

 the simplest living organisms known, namely the 

 Protozoa, Protophyta and Bacteria. It may be true 

 that these simple unicellular organisms are primitive, 

 in the sense that they represent a very early stage in 

 animal evolution when the animal or plant body was 

 composed of a single cell, but this early stage is 

 very far removed from the origin of life itself. The 

 Protozoa comprise a great variety of animal forms, 

 characterized by comparative simplicity of structure 

 and by the fact of their body being composed of 

 a single cell. Such a Protozoon as Amoeba (Fig. 5 A), 

 which consists of a small mass of naked foam-like 

 protoplasm in which a nucleus of more solid con- 

 sistence is differentiated, represents the lowest grade 

 of animal organization known to us. The whole 

 surface of the body is capable of protrusion for the 

 purposes of locomotion or for engulfing food; an 

 excretory vacuole can be formed at any part of the 

 body, and reproduction is effected simply by the 

 division of the body into two, the division of the 



