1922] on Microscopic Parasites and their Carriers 513 



present in fish, frogs and other cold-blooded aquatic vertebrates. It 

 is clear that the trypanosomes of fish cannot be conveyed from fish 

 to fish by biting insects. Another blood-sucking invertebrate, the 

 leech, is responsible for their transmission. When the leech feeds it 

 does so by means of a proboscis armed with teeth, and this proboscis 

 is lodged in a sheath or pouch, through the opening of which it can 

 be protruded. The trypanosomes have availed themselves of the 

 presence of this proboscis sheath. Taken into the stomach of the 

 leech when it feeds on an infected fish or frog, the trypanosomes 

 multiply there, and finally migrate forwards along the oesophagus 

 and proboscis. They pass out of the month at the end of the pro- 

 boscis and enter the proboscis sheath, where they continue to multiply 

 and wait their opportunity. When the leech feeds the circular open- 

 ing of the proboscis sheath is placed against the skin of the fish, and 

 the proboscis with its teeth inflicts the wound. The fluid in the 

 proboscis sheath, which may be swarming with trypanosomes, comes 

 in contact with the wound and the trypanosomes enter the body of 

 the fish. The trypanosomes in this instance have not availed them- 

 selves of the salivary glands but of the sac-like proboscis sheath. 



In the three examples I have given it will be seen that infection 

 is spread by the biting parts of the invertebrate, and that the para- 

 sites develop in the anterior region of the body. This type of 

 development has been termed a development in the anterior station. 

 In the examples which I will now give the development is at the 

 opposite end of the body — a development in the posterior station — 

 and the method of transmission is correspondingly different. 



The common rat very frequently harbours a trypanosome in its 

 blood which is conveyed from rat to rat by fleas. Now, it might 

 have arisen that these trypanosomes would avail themselves of the 

 salivary glands of the flea, as in the case of the tse-tse fly, but this 

 is not so. The flea has the peculiar habit, which is shared by many 

 insects, of not ceasing to feed when its stomach is full. In order to 

 accommodate the blood which it is greedily sucking it has constantly 

 to empty its intestine. If one watches a flea feed it will be noticed 

 that every few minutes it ejects from its hinder end considerable 

 quantities of blood. In this manner a flea is able to enjoy a pro- 

 longed feed which may continue for an hour or more. Hence the 

 numerous spots on the sheets if one is unfortunate enough to have 

 to sleep with one of these insects. 



The trypanosome of the rat, when taken up by the flea, multi- 

 plies in the flea's intestine, and finally passes backwards to the 

 rectum, where multiplication is continued. When such a flea feeds, 

 every time it ejects a droplet of blood some of the trypanosomes in 

 the rectum are carried with it, and are deposited on the skin or fur of 

 the rat. If a white rat which is infested with fleas is watched it will 

 be seen that the insects tend to congregate on the back of the rat 

 above the tail. Such a rat will go to sleep, and the fleas will seize 



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