IMMUNITY AND ADAPTATION. 143 



Certain parasitic animals, as the leech and anchylostoma, live 

 entirely or partially by taking into their digestive system the 

 blood of the host. In regard to anchylostoma it is not quite 

 certain whether it sucks the blood of the host directly or whether 

 it obtains the blood together with parts of the mucous mem- 

 brane of the internal canal which it inhabits. Blood of the host 

 certainly forms an important part of the food of the parasite. 

 Both of these worms contain in the anterior part of their body, 

 substances which strongly inhibit the coagulation of the blood. 

 Thus the blood is kept in a liquid state and the sucking of the 

 blood and probably also its digestion and resorption are rendered 

 much easier. This was demonstrated in the case of the leech 

 many years ago by Haycraft and recently also in the case of an- 

 chylostoma. Here again we have to deal with a process which 

 to some degree is specific, inasmuch as the substance which has 

 such a powerful action in inhibiting the coagulation of the blood 

 of the host is absolutely without influence on the coagulation of 

 invertebrate blood. And there are even certain indications tend- 

 ing to show that this substance does not act equally strongly on 

 all vertebrate blood, inasmuch as Sabbatani found that a similar 

 substance in another blood-sucking animal, ixodes ricinus, acts 

 much more strongly on dog blood, the blood which is sucked by 

 this tick, than on rabbits' blood. I myself found in one experi- 

 ment made that the substance which is present in anchylostoma, 

 inhibiting the coagulation of the blood of the dog, was powerless 

 towards the blood of guinea pigs. 1 



The fact that leech extract is without effect on the blood of the 

 lobster makes it furthermore more probable that it is also without 

 power upon the blood of the leech itself. 



Snake venom, a most powerful poison for many vertebrates, is 

 almost harmless for snakes, themselves. This is as Phisalix found 

 at least partially due to the presence of an antitoxin in the blood 

 of the snakes. The blood of these animals may therefore contain 

 toxin and antitoxin side by side. This immunity of snakes against 

 their own poison is of great significance, otherwise an injury of 

 their tongue or any other part of their body by the teeth of the 

 animal would be fatal. 



A toxin similar to snake venom is contained in the abdominal 



1 This experiment needs to be repeated. 



