THE PROTOZOA THE DAWN OF LIFE 15 



quite recent years scientific investigation has been closely applied ; 

 but since this class has been recognised to contain organisms which 

 induce malarial fever in man, and various other diseases in man 

 and animals, there has been no lack of workers, and much success- 

 ful investigation has been carried out. To the researches of von 

 Siebold, Kolliker, and van Beneden we owe our earliest accurate, 

 though partial, knowledge of the class ; while some thirty years 

 ago Sir E. Ray Lankester began the study of those species which 

 live in the blood, an epoch-making piece of biological investigation 

 of unique and immense importance to mankind. The work be- 

 gun by these pioneers has been carried on by Manson, Ross, 

 Minchin, Grassi, Laveran, Blanchard, Leger, Cuenot, Schaudinn, 

 and many other distinguished English and Continental workers. 

 The result of the work of these numerous investigators has shown 

 the Sporozoa to be of very widespread occurrence, exclusively 

 parasitic in habit, and infesting the internal organs or tissues of 

 animals belonging to nearly every class and order of the Metazoa. 1 



The greatest diversity of structural and developmental char- 

 acters are exhibited by the Sporozoa, and there is a general, 

 though not universal tendency for each species to be parasitic 

 on a particular species of host, and to be confined to certain organs 

 or tissues of that host. The effects produced by the Sporozoa 

 upon the animals they infest vary greatly, in many cases, possibly 

 in most, causing no apparent discomfort or injury to the health 

 or vitality of their host, but in others producing most dangerous, 

 not infrequently fatal diseases, and ravaging epidemics. Wide 

 limits as regards size are reached by the different species, some 

 being so minute that several can be contained in a single blood- 

 corpuscle, while others, like the Gregarine Sporozoon (Porospora 

 gigantea), parasitic in the lobster, are clearly visible to the naked eye. 



Considered as a group, the Sporozoa will be found to possess 

 in common certain very characteristic features. Thus food- 

 vacuoles or contracting vacuoles are never present, the nutri- 

 ment consisting of the juices of the host, and therefore always of 

 a fluid nature, being absorbed by diffusion through the cuticle of the 

 parasite's body ; while, where flagella or pseudopodia are present 

 they exist essentially as organs of locomotion and not of nutrition. 



1 From the Greek many-celled or multicellular animals, as distinct from the 

 Protozoa or unicellular animals. 



