292 PROTOZOA AND METAZOA CHAP. 



for instance, thread-worms spring from Euglenae or Infusoria 

 from Bacteria ? To this it is sufficient to answer that the 

 evolution of one form from another takes place by a series 

 of slow, orderly, progressive changes going on through a 

 long series of generations (p. 222) ; whereas heterogenesis 

 presupposes the casual occurrence of sudden transformations 

 in any direction i.e., leading to either a less or a more highly 

 organised form and in the course of a single generation. 1 



Each of the organisms which we have studied in this 

 and the two previous chapters consists of a single cell or 

 in the case of Carchesium and Epistylis of a colony of cells 

 to a large extent independent of one another. They are 

 therefore placed in the lowest primary division of the animal 

 kingdom the phylum Protozoa (p. 220). This phylum is 

 subdivided into a number of classes, examples of certain of 

 which we have examined. Those in which, like the Amoeba, 

 the amoeboid form is predominant constitute the class 

 Rhizopoda : those in which, like the Monads and Euglense 

 (Flagellatd), the flagellate form is predominant are often 

 included with the ciliated forms (Ciliata) such as Para- 

 mcecium, Vorticella and Opalina in a single class, the 

 Infusoria : and those in which, like Monocystis, the encysted 

 form is predominant, are known as the Sporozoa. 



The animals above the Protozoa are placed, as we have 

 seen, in a number of different phyla, but as they are all 

 multicellular they are often spoken of collectively as the 

 Metazoa, one of the simplest of which we must next 

 examine. 



1 Apart from such continuous variations, others, which may be 

 described as discontinuous, do sometimes appear with apparent sudden- 

 ness, but not to the extent which would be required by the theory of 

 heterogenesis. 



