736 MEDICAL ENTOMOLOGY 



to some other organ. The first is apparently what happens in the case 

 of Spirochaeta obermeiri, which, according to Nicolle, Blaizot and Con- 

 seil, is not found in the alimentary tract of the louse after the lapse of a 

 few days after the infecting feed, and is not found in the salivary glands 

 or ovaries at any time. The only exit from the body in such a case is by 

 rupture of the body wall, and according to the above observers this is what 

 actually occurs ; the louse is crushed by the person who harbours it, and 

 the liberated spirochaete finds its way into the blood through an abrasion. 

 It is possible that some protozoa, especially such as have for their hosts 

 the more sedentary parasites, may find their way into the tissues of the 

 vertebrate by a similar route ; the presence of the invertebrate host irri- 

 tates the vertebrate, which endeavours to free itself, and in doing so either 

 swallows the invertebrate or ruptures its integument with its teeth, thus 

 enabling the protozoon to enter either the buccal or the gastric mucous 

 membrane. According to Strickland, rats may become infected with 

 Trypanosoma lewisi by swallowing its invertebrate host, the rat flea. 

 The case of Filaria bancrofti, and probably of many other filariae, is 

 rather different. Here the infective stage is free in the haematocoele, 

 but passes forwards to the proboscis, and is set free at the seat of 

 the wound, in some manner which has not yet been determined. 



Should the parasite not remain free in the haematocoele, it must pass 

 to one or other of the organs. How readily this may occur will be under- 

 stood at once by recalling the nature of the haematocoele and of the 

 circulation. The haematocoele is little more than a potential space, all 

 the organs of the abdomen being tightly packed together exactly as in 

 the case of more complex animals ; the Malpighian tubes, which are 

 diverticula of the gut, are usually very long, and are twined in and out 

 throughout the cavity ; the same is often the case with the salivary 

 glands, which, as in Glossina, may be almost entirely abdominal. A 

 glance at the sections figured on Plate XXV will show at once how 

 intimate are the relations of the abdominal organs to one another, and 

 how easily a parasite which retained its motility after passing through 

 the wall of the gut might penetrate any one of them. Further, the 

 blood in this space is kept in a state of constant movement by the action 

 of the heart, which drives it forward through the aorta and draws it back 

 again through the pericardial space, so that no matter at what point 

 the organisms penetrate the wall of the gut they will, unless they 

 penetrate other organs and so anchor themselves at once, be distributed 

 all over the cavity. 



The only organs in which parasites have actually been found are the 



