2 THE PROTOZOA 



cultivated zealously even by many who are not professed biologists, 

 with the result that our knowledge of these organisms has made 

 very great strides in the last two decades, and is advancing so 

 rapidly that it becomes increasingly difficult for any single person 

 to keep pace with the vast amount of new knowledge that is pub- 

 lished almost daily at the present time. 



While the attention that is now focussed upon the Protozoa has 

 led to a most gratifying increase of scientific and medical knowledge 

 concerning particular forms, it tends frequently to a certain vague- 

 ness in the notions held with regard to the nature and extent of 

 the group as a whole. This is owing largely to the fact that many 

 are now attracted to the study of the Protozoa whose aims are 

 purely practical, and who investigate only a limited number of 

 species in minute detail, without having an adequate foundation 

 of general knowledge concerning other forms. Hence io is important 

 to attempt to frame a general definition of the Protozoa, or at least 

 to characterize these organisms in such a way as to enable a dis- 

 tinction to be drawn between them and other primitive forms of 

 life. This object may be attained logically in two ways either by 

 considering the distinctive characters of the group, or by enu- 

 merating the types of organisms which constitute it ; in more 

 technical phraseology, by determining either the connotation or the 

 denotation of the term Protozoa. To attempt this task will be 

 the object of the present chapter. 



The name Protozoa was first used in 1820* as an equivalent 

 of the German word Urthiere, meaning animals of a primitive or 

 archaic type. This fitting designation superseded rapidly the older 

 term Infusoria (Infusionsthiercheri), used to denote the swarms 

 of microscopic organisms which make their appearance in organic 

 infusions exposed to the air. The word Infusoria is now em- 

 ployed in a restricted sense, as the name of one of the principal 

 subdivisions of the Protozoa (pp. 12 and 430). 



The first attempt at a scientific definition of the Protozoa was 

 given by von Siebold, who defined them, from a strictly zoological 

 standpoint, as unicellular animals. This definition, or a modifica- 

 tion of it, is still the one given, as a rule, in zoological textbooks ; 

 and from this time onwards the animal kingdom was subdivided 

 universally into the Protozoa and the Metazoa. The Protozoa, 

 as organisms in which the individual is a single cell, are regarded 

 as those which come first (Trpwros) in the ascending scale of animal 

 life, or in the course of organic evolution ; the Metazoa, in which 

 the individual is an organism composed of many cells, come after 

 the simpler forms of life in rank and time. 



* For the detailed history of the growth of scientific knowledge of the Protozoa, 

 see Biitschli (2), pp. i-xviii. 



