508 Dr. C. Morley Wenyon [March 3^ 



sucking invertebrates. Having' been taken into the intestine of such 

 an invertebrate they may pass unchanged through its intestine and 

 come into the outside world, where they wait till they are eaten by 

 another vertebrate host. But having once entered the invertebrate 

 their chance of survival will be greater if, instead of simply passing 

 through its intestine in a casual manner, they establish themselves 

 and multiply as true parasites. They may produce resistant forms 

 which are passed continuously in the dejecta so often deposited on 

 the skin. It is evident that in this position, as, for instance, on 

 contaminated fingers, it is easy for them to be eaten by the vertebrate. 

 Or they may so regulate their lives in the invertebrate that they come 

 into association with the biting organs, and are inoculated to another 

 vertebrate when a second feed of blood is taken later on. 



The parasites which are taken up from the blood by blood-sucking- 

 invertebrates can therefore re-enter another vertebrate by two methods. 

 They may find their way to the structures which are related to the 

 biting organs and be inoculated when biting again takes place, or 

 they may appear in the dejecta of the invertebrate and enter the body 

 as a result of co: tamination of the skin. The former is known as the 

 inoculative, and the latter as the contaminative method of infection. 



It will be evident that as the habits of invertebrates vary 

 considerably, unless the parasites adapt themselves, or rather become 

 adapted by the process of survival of the fittest, to those particular 

 habits, their chances of transmission are very small indeed, and I 

 want to show by a few examples how this kind of adaptation takes place. 



The best known illustration is to be found in the case of the 

 parasites of malaria of man which are transmitted by anopheline 

 mosquitoes. The malarial parasites of man were probably derived 

 in the first place from intestinal parasites, like coccidia, which gave 

 up a life in the wall of the intestine for one in the red blood 

 corpuscles. They multiply in the blood, and eventually produce 

 resistant forms which in their original intestinal habitat would have 

 escaped to the exterior with the dejecta. As these resistant forms 

 are now in the blood it is difficult, if not impossible, for them to get 

 into the intestine, while it constantly happens that they are taken 

 up by blood-sucking mosquitoes. Accordingly they have adapted 

 themselves to a life in the mosquito, but instead of passing through 

 the mosquito's intestine to the exterior, as might have been the case, 

 and possibly did occur, during the course of evolution, they multiply 

 in the mosquito's body and produce a large number of small sickle - 

 shaped bodies, known as sporozoites, which are scattered all through 

 the tissues. Now the mosquito, in common with other insects which 

 feed on blood, has the habit of injecting saliva from its salivary 

 glands into the wound at the commencement of its feed. It is 

 probable that this saliva, which is an irritating fluid, causes the blood 

 supply to the area around the wound to be increased so that the 

 mosquito can more readily obtain the blood it desires. The sporo- 



