346 CARRIERS OF DISEASE 



shaped), or even spherical particles (micrococci) and 

 only in favourable conditions arrest their self-division so 

 as to grow for a time into the thread-like or filament 

 shape. Often these filaments are not straight, but spirally 

 twisted, and are called " spirilla." Some of them are 

 blood parasites, but the larger number attack the tissues, 

 and others occur in the digestive canal. 



The parasitic disease-producing protozoa, on the other 

 hand, are of softer substance, often have the habit of 

 twisting themselves in a corkscrew-like manner, and 

 usually are provided with an undulating membrane or frill, 

 as well as with one or with two whip-like swimming 

 processes (the latter are present also and are often nume- 

 rous in the actively swimming phases of bacteria), and have 

 a more complicated life-history. They divide, as a rule, 

 longitudinally and not transversely, and pass from one 

 " host " to a second, where they assume distinct forms 

 males and females, which conjugate and break up (each 

 conjugated or fused pair) into a mass of very numerous, 

 excessively minute, young. The disease-producing pro- 

 tozoa of this kind are frequently parasitic in the blood of 

 man and animals, and were only recently recognised, 

 after the disease-producing bacteria of many kinds had 

 been thoroughly studied. These animal microbes are 

 often spoken of as " blood-flagellates " or haemo-flagellata, 

 and the larger kinds are called " Trypanosomes," or 

 " screw-form parasites," whilst a series of more minute 

 ones are called " Piroplasma," or " pear-shaped parasites." 

 Many, but not all, are found during a certain period of 

 their life, actually inside the corpuscles of the blood. 

 The fact that many of these blood-flagellates (if not all) 

 have, besides their life in the blood of one species of 

 animal, a second period of existence in the juices or the 

 gut of another animal, has made it very difficult to trace 

 their migrations, since in the second phase of their history 



