22 THE PROTOZOA 



(T. equiperdum) passes from one host to another during coitus, and 

 the transmission of the parasite of syphilis is another instance. 



Speaking generally, and excluding for the moment those cases 

 in which the transmission is brought about by means of an inter- 

 mediary host, the propagative phases of the parasitic Protozoa 

 take the form of inactive, resting stages in which the body of the 

 parasite is protected against adverse external conditions by tough 

 protective membranes. In the form of resistant cysts or spores, 

 the parasites in a dormant state offer a passive and inert resistance 

 to the elements ; they are disseminated like seeds, and they ger- 

 minate when they reach a suitable environment, but not till then. 



Many, perhaps the majority of parasitic Protozoa, occupy posi- 

 tions in the body of the host whence the propagative phases can pass 

 without difficulty to the exterior. This is the case when the para- 

 site is lodged in organs which have ducts or passages leading directly 

 or indirectly to the exterior such as, for instance, the digestive 

 tract and its dependencies, or the urinary organs and ducts. In all 

 such cases the propagative stages of the parasite pass harmlessly 

 to the exterior. The host may in this manner get rid entirely of its 

 parasites, without, however, necessarily acquiring immunity to 

 fresh infections ; or, on the other hand, the parasite may keep up its 

 numbers in the host by continual multiplication to produce a stock 

 from which are sent forth incessant relays of the propagative phases 

 destined to infect new hosts. In the majority of parasitic Pro- 

 tozoa the relations to the host are of this type, and the parasites are 

 neither lethal nor pathogenic tc any great extent. 



On the other hand, there are many instances in which parasitic 

 Protozoa occupy a position in the body of the host whence escape 

 by anatomical channels is not possible. This is the case when the 

 parasite inhabits some closed space in the body, such as the ccelome 

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

 deeply-situated cells or tissues of the body. In some cases w r here 

 natural means of exit from the body occur, they may be unsuitable 

 for the dissemination of the parasite, as in the case of those forms 

 parasitic in the genital organs of one sex of the host. In cases of 

 this kind there are at least six known methods whereby parasitic 

 Protozoa are disseminated and 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 Myxobolidce, are disseminated ; they 

 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 suppurate, 

 and so set free the cysts or spores of the parasite. This, again, is 



