520 Dr. C. Morley Wenyon [March 3, 



the louse is the culprit. There is some evidence that the spirochete 

 in the louse may pass through the egg, as in the case of the tick, 

 but there is a more common mode of transmission. Though 

 such a persistent ectoparasite of man under filthy conditions, and 

 though difficult to eradicate when there is poverty and overcrowding, 

 the louse is a very delicate creature which is easily damaged. The 

 spirochetes which the louse has sucked up in the blood multiply 

 rapidly in the body of the louse till all the body fluids are crowded 

 with them. They extend into every part of the body, including the 

 legs and antennae. The violent and constant scratching in which a 

 lousy individual indulges frequently damages the lice so that their 

 legs are broken off. Fluid filled with spirochetes exudes from the 

 stump of the broken leg on to the skin, and this makes its way into 

 the puncture wound inflicted by the louse, or into the erosions caused 

 by the nails of the unfortunate individual himself. Infection is 

 spread from man to man by the damage caused to the louse. From 

 the point of view of the spirochaete this is a highly satisfactory 

 arrangement, but hardly so for the louse. Still less is the method of 

 transmission which occurs in some cases, as, for instance, that of the 

 haemogregarine of the dog — a parasite which lives in the white blood 

 corpuscles, and which, like the piroplasma of the same animal, is 

 conveyed by ticks. The haemogregarines gain entrance to another 

 dog probably by the dog actually devouring the infected tick. 



Time prevents me from entering into this most interesting 

 subject in greater detail. I hope I have made it clear how parasites, 

 which originally passed directly from one host to another by means 

 of resistant stages protected by cysts which contaminated water, 

 have, in the course of evolution, changed their habitat in the body, 

 passing from the intestine to the blood, and how it is in consequence 

 of this change that they have been obliged to seek some other means 

 of transit. Quite naturally, blood-sucking invertebrates were utilised 

 to this end, but the behaviour of any parasite in the invertebrate 

 had of necessity to be closely adapted to the particular habits of the 

 invertebrate it employed. 



I have spoken throughout of the parasites " availing themselves " 

 of this and that peculiarity, but I have merely done so for the sake 

 of convenience of speech. I do not wish to imply that the parasiks 

 are in any way conscious of what is going on. Once in the inverte- 

 brate they multiply and spread through the body indiscriminately. 

 It is only those which happen to arrive at a situation whence they 

 can re-enter a host which will be transmitted. It may be that the 

 fortunate ones have some peculiarity, possibly of a chemical nature, 

 which causes them to be drawn there by a chemiotactic action. If 

 this be so, then the offspring of the lucky few will probably inherit 

 this quality to a higher degree, so that the next time they infect the 

 invertebrate a still smaller number will go astray. Finally, there 

 will be developed that peculiarity, so characteristic of parasites 



