CHAPTER II 



PHYLUM PROTOZOA 



TnE Protozoa are distinguished from all other animals (l) by 

 the fact that when they produce germ cells or gametes the whole 

 animal breaks up into them, (2) by the fact that the protoplasm of 

 the body is never differentiated into tissues nor exhibits cellular 

 structure (see p. 30) \ The higher animals are often grouped under 

 the name Metazoa (Gr. /xera, after; woi/, an animal) in order to 

 contrast them with the Protozoa, but whereas the Protozoa, since 

 they have a common structural ground-plan, constitute a phylum in 

 the sense defined in the last chapter the same is by no means true 

 of the Metazoa. Hence the name Metazoa does not denote a 

 phylum but is a mere convenient collective term. 



The term Jnvertjsjmtta is also a mere collective name ; it 

 is employed to designate all animals which 4p.,npt_belon^_to the 

 phylum Vertebrata. Like the name Metazoa its convenience in 

 promoting terseness of expression is its only justification. The 

 Protozoa are thus Invertebrata and the Vertebrata are Metazoa. 



The phylum Protozoa includes the simplest and lowest 

 members of the animal kingdom. With few exceptions the members 

 of this phylum are too small to be seen by the naked eye, and yet 

 many of them are of great importance in the economy of nature. 



In order to fix our ideas we may select one of the simplest 

 Protozoa as a type for examination. Amoeba y some- 

 times called the Proteus animalcule, from its power 

 of continually changing its shape, is found in the mud at the 

 bottom of ditches, ponds and pools of stagnant water. There are 

 several species varying somewhat in size included under the 

 generic name Amoeba, all of them, however, are so small as to 



1 These statements are true of the vast majority of animals classed as 

 Protozoa. The exceptions are for convenience classified as Protozoa, but it 

 seems to the authors that in the light of a fuller knowledge they may turn out 

 to be survivors of that great series of forms which must, if the evolution theory 

 be true, have intervened between the Protozoa and the Metazoa. 



