10 THE president's ADDRESS. 



the parasite is lodged in organs which have ducts or passages 

 opening directly or indirectly to the exterior : for instance, in the 

 digestive tract and its dependences, such as the liver, saliv^ary 

 glands, malpighian tubules of insects, etc. ; or the urinary organs 

 and ducts. In all such cases the propagative phases of the 

 parasite pass harmlessly to the exterior. The host may in this 

 manner get rid entirely of its parasites, without however 

 necesjsarily acquiring immunity to fresh infections ; or, on the 

 other hand, the parasite may keep up its numbers in the host by 

 *' endogenous" multiplication while at the same time it is con- 

 tinually sending forth the propagative phases destined to infect 

 new hosts. In the majority of Protozoan parasites the relations 

 to the host are of this type, and the parasites are neither lethal 

 nor pathogenic to any great extent. 



On the other hand, there are many instances in which 

 Protozoan parasites occupy a position in the body of the host 

 whence escape by anatomical channels is not possible, or if 

 possible, not suitable. This is the case when the parasite in- 

 habits some enclosed space in the body, such as the coelome or 

 general body-cavity, or the blood-system ; or when it attacks 

 deeply-seated cells or tissues of the body. In such cases there are 

 at least six known methods whereby the parasite is disseminated 

 or transferred to fresh hosts. 



1. The resistant stages of the parasite may be set free by the 

 death and decay of its host. This appears to be the manner in 

 which some of the tissue-infecting parasites of the order Myxo- 

 sporidia, especially the family Myxoholidae, are disseminated. 

 These organisms are for the most part parasites of fishes, and are 

 often very deadly in their effects. 



2. The parasite may cause tumours and ulcers, which sup- 

 purate and so set free the spores or cysts of the jDarasite. This 

 again is an eftect commonly produced by tissue-parasites such as 

 the Myxoholidae. In such cases the parasite is always patho- 

 genic and frequently lethal to the host. 



3. The parasite remains in the host until the latter is eaten 

 by some animal which preys upon it. The propagative phases 

 of the parasite are able, however, to resist digestion by the 



