152 STATUS OF PROTOZOA 



of Entamoeba histolytica, but at present there is no reliable evidence that 

 these actually exist. A drug which fails to act on a parasite may do so 

 because of some peculiarity on the part of the host. The whole subject 

 of the method of action of drugs in the treatment of Protozoal infections 

 is exceedingly complicated, and opens a field for extensive investigations. 

 A very instructive resume of the subject has been made by Dale (1924). 



STATUS OF THE PROTOZOA IN THE ANIMAL KINGDOM. 



It is usual to regard the Protozoa as constituting a Phylum wdiich 

 corresponds in status to one of the various Phyla, such as the Mollusca, 

 Arthropoda, Vertebrata, etc., into which the rest of the animal kingdom 

 is divided. This is the view adopted by most zoologists, but Dobell and 

 O'Connor (1921) have recently expressed the view that the Protozoa 

 constitute a group of organisms which has a status equal to the rest of 

 the animal kingdom. According to Dobell's contention, discussed earlier 

 in this work, the Protozoa are non-cellular animals, while the rest of the 

 animal kingdom includes all cellular animals. On this account he 

 divides the animal kingdom into two sub-kingdoms — the Protozoa and 

 the Metozoa. Such a distinction may still be admitted, though there 

 would be less reason for its recognition if the generally accepted view were 

 held that the Protozoa are unicellular, and not merely non-cellular animals. 

 Dobell, having raised the Protozoa to the rank of sub-kingdom, raises 

 to the status of Phyla the various classes in which they are divided. For 

 purposes of this work, however, it is unnecessary to discuss this very 

 intricate subject, and, following the more orthodox view, the Protozoa 

 will be still regarded as constituting a Phylum. 



