THE MODES OF LIFE OF THE PROTOZOA 21 



the " propagative phases," which as a rule do not multiply further 

 in the host in which they are produced, but await their chance of 

 being transferred to a new host ; and if such a chance be not given 

 to them, they die off and are replaced by fresh propagative forms 

 from the stock (see further below, Chapter IX., p. 166). 



So long as the nutritive or multiplicative function is the most 

 important one in the life of a parasite, and until it has matured its 

 propagative phases, the death of the host is the greatest disaster 

 that can befall it. The ideal host, from the point of view of a para- 

 site, is one that is " tolerant " to it that is to say, one that can 

 support the presence of the parasite and koep it supplied with the 

 nutriment it requires, without suffering in health or vigour to any 

 marked extent. When once, however, the parasite has made the 

 necessary provision for propagating the species, the life or death 

 of the host may become a matter of indifference to the parasite, 

 or may even in some cases be necessary for the dissemination of the 

 offspring. This will be apparent from a consideration of the methods 

 by which parasitic Protozoa infect new hosts. 



The passage of a parasite from one host to another includes two 

 manoeuvres : the passing out from the first host, and the passing 

 into the second. Primitively it may be supposed that this migra- 

 tion was effected simply by the unaided efforts of the parasite itself 

 that is to say, that the active motile parasite would force its way 

 out of one host, move freely in the surrounding medium, and sooner 

 or later attack and penetrate a fresh host. This primitive method 

 of transference doubtless occurs in many cases, especially amongst 

 epizoic forms (e.g., Ichthyophthirius, p. 450). In the case of entozoic 

 parasites its occurrence is less common, but it is found in a certain 

 number of cases. The young stages of many Acinetaria, parasitic 

 in Ciliata, probably seek out their hosts and penetrate into them ; 

 after a period of juvenile parasitism they leave the host's body and 

 become free-living, non-parasitic organisms. Active migration of 

 this kind, however, is very rare amongst entozoic parasites. In the 

 first place, the conditions of life within a living body, in the midst of 

 organic fluids, are so different from those in the open water, whether 

 salt or fresh, that it is hardly to be expected that a delicate unicellular 

 organism adapted to the one mode of life could stand the sudden 

 change to the other. In the second place, it is clear that active 

 migration of parasitic Protozoa could only be effected when the host 

 is an aquatic animal, and not when it leads a terrestrial life. The 

 only instances of active migration known with certainty to occur 

 in the case of Protozoa parasitic on terrestrial animals are those 

 in which the parasite can penetrate a mucous membrane, and is 

 thus able to pass from one host to another when two such surfaces 

 are in contact. In this way the trypanosome of dourine in horses 



