i8 THE PROTOZOA-THE DAWN OF LIFE 



converted many a fever-haunted district into a healthy, prosper- 

 ous neighbourhood. Wherever the Anopheline mosquito has been 

 stamped out, malaria has disappeared, and the health, vigour, 

 and prosperity of the people increased. The life-history of the 

 Anopheline mosquito, and the methods employed for its exter- 

 mination, will be dealt with in a later chapter. 



The first Sporozoa to be closely observed and studied were 

 the Gregarines, on account of their comparatively large size. 

 They are to be found living in the internal organs of various 

 insects, crustaceans, echinoderms, and worms, and the majority 

 appear to exert no ill-effects upon their host, feeding upon the 

 liquid food of the host as it exists in the intestine. In most 

 species the body is rather elongated and flattened, in the species 

 which infests the intestine of the so-called " meal-worm " l some- 

 what resembling an Indian club in shape. When about to mul- 

 tiply, the Gregarine contracts its body into a ball-shaped mass 

 and becomes encysted. The nucleus and protoplasm within the 

 cyst divide into numerous minute spores, which ultimately make 

 their escape and grow into new Gregarines. 



We have seen that in the course of their life-history some of 

 the Protozoa we have had under consideration pass in the process 

 of multiplication through a spore stage characterised by the 

 presence of a slender lasher or flagella. In a great number of 

 Protozoa, however, we shall find that this flagellate condition 

 of the cell is not a transitory one, but is the permanent condition 

 of the adult organism. These forms are included in the class 

 Flagellata 2 of the Protozoa, and comprise a very heterogeneous 

 assemblage of organisms, very interesting and very puzzling to 

 the student, for in this, more than in any other class, those formal 

 distinctions which are commonly drawn between the animal and 

 vegetable kingdoms disappear. Nevertheless the organisms com- 

 prising this class possess in common certain characteristic traits 

 of organisation, such as a single nucleus, one or more contractile 

 vacuoles, and one or more flagella, which link them together. 

 The vast amount of biological investigation which has been accom- 

 plished has greatly widened the field of knowledge concerning 



1 " Meal-worms " are the larvae of Tenebrio molitor, a small beetle all too common 

 in granaries and flour mills. 



* The Flagellata as here described correspond to Butschli's group of the Masti- 

 gophora. 



