24 THE PROTOZOA 



need only be noted here that in such cases resistant spores or cysts 

 become unnecessary and superfluous for the propagation of the 

 parasite, and tend to disappear from its developmental cycle. 



6. In some cases the parasite may penetrate the ovary of its host, 

 pass into the ova, and thus infect the embryo and the next genera- 

 tion. Transmission of this kind is known in a certain number of 

 cases ; it is never the sole method of transmission, but is always 

 supplementary to other methods. For instance, in " pebrine " of 

 silkworms, caused by Nosema bombycis, the spores of the parasite 

 are liberated in the ordinary way from the caterpillar either with the 

 faeces or by its death, and are then eaten accidentally on the leaves 

 by other silkworms ; but a certain number of the parasites pene- 

 trate into the ovary and form spores, which pass through the pupal 

 and imaginal stages of the host into the next generation of silk- 

 worms, which are born infected. In this way the parasite is able 

 to tide over the winter season, when the ordinary method of infec- 

 tion would be impossible. The blood-parasites of the genus 

 Piroplasma (p. 384, infra) afford another example of gerniinative 

 infection in the ticks which transmit them. 



To turn now to the methods by which parasitic Protozoa pene- 

 trate into new hosts ; there are four known methods, which, after 

 what has been said, can be summarized very briefly. The com- 

 monest is the method of casual or coiitaminative infection, where 

 the host infects itself accidentally by taking up the propagative 

 phases of the parasite from its surroundings most usually by way 

 of the mouth, with the food, but it may be by way of the respira- 

 tory organs. Other modes of infection are the contagious, as in 

 dourine, already mentioned ; the inoculative, as in malaria and 

 other diseases caused by blood-parasites ; and the so-called " heredi- 

 tary " or " gerniinative " method, as in Nosema bombycis and other 

 cases. 



From the foregoing summary of the methods by which parasitic 

 Protozoa are propagated from one host to another, it is clear that 

 there are very few cases in which it is of direct advantage to the 

 parasite to cause the death of its host. Even where it is necessary, 

 for the propagation of the parasite, that the host should be destroyed 

 by some other animal, as in the case of the Monocystis of the earth- 

 worm, the interests of the parasite are not furthered, and may, 

 indeed, be damaged, if it cause disease or death to the host. In 

 the case of blood-parasites, transmitted by the inoculative method, 

 it may be necessary for the propagation of the parasite that the 

 required phases should be sufficiently abundant in the blood of the 

 vertebrate host to insure the invertebrate host becoming infected 

 when it sucks the blood ; then large numbers of the parasite may be 

 detrimental to the well-being of the host to a greater or less extent, 



