518 Dr. C. Morley Wenyon [March 3, 



the faeces which the bug passes when it feeds. This material con- 

 taining trypanosomes is either rubbed or scratched into the wound 

 inflicted by the bug, or is carried on the fingers to the mouth in such 

 a way as to bring about infection of the human being. The bug 

 remains infective for the rest of its days. 



We thus find from the examples we have just been considering 

 that the parasites in the invertebrate tend to develop in the anterior 

 station when infection is to be inoculative, and in the posterior station 

 when it is contaminative. 



There is another class of blood-sucking invertebrates which differ 

 from those we have been talking about in that they do not feed 

 repeatedly. A mosquito or a tse-tse fly feeds possibly every day, 

 while in the intervals between its feeds it lives elsewhere. Now in 

 the case of ticks there is a different state of affairs. The young tick 

 hatched from the egg fixes itself to the skin of its host by its 

 proboscis, and may remain there for the rest of its days, only 

 loosening its hold towards the end of its life to fall on to the ground, 

 where it lays eggs and dies. Some ticks pass their larval stage on one 

 vertebrate, the nymph stage on a second, and the adult stage on a 

 third. Others have only two hosts. The ticks are known, there- 

 fore, as one-host ticks, two-host ticks and three-host ticks. In the 

 case of a one-host tick if a parasite were to avail itself of this host 

 it might at first sight seem impossible for such a tick to transmit an 

 infection, for it lives on but one host, and leaves it only to lay eggs 

 and die. It does not visit a second host to which it might convey a 

 parasite which it had taken up. But we shall see that such a trans- 

 mission is still possible. There is a group of parasites known as 

 piroplasmata which in many respects resemble the malarial parasites 

 of man. They live in the red blood corpuscles, where they multiply. 

 They do not, however, produce the characteristic brown pigment 

 which is found in the malarial parasites. They occur in cattle, 

 horses, sheep, dogs and other animals, and give rise to serious diseases 

 known as red-water fever and malignant jaundice. These organisms 

 are taken up from the blood by ticks, where they multiply and finally 

 invade the ovaries. When the tick leaves its host and lays its eggs 

 the latter have already the parasites within them, so that the young 

 ticks or larvae which develop in the egg become infected. The para- 

 sites multiply in the young ticks, and when they attach themselves to 

 another host they transmit the infection. It is not yet clear whether 

 the parasites are injected with the saliva of the tick, or whether they 

 are deposited on the skin in its dejecta, or in secretions from certain 

 glands; but the interesting fact is that the parasite has adapted itself 

 to a life in the egg of the tick so that it may be transmitted by the 

 succeeding generation. In the case of two-host ticks and three-host 

 ticks this passage of the parasite through the egg still takes place, 

 but it also happens that the narasites taken up by the first stage or 

 larva of the tick may be transmitted when one of the succeeding 



