INTRODUCTORY 7 



the propagation of all the various protozoa which cause 

 disease, as in some, such as the coccidia and Amoeba 

 coll, part of the development takes place in earth or 

 in water without an alternative host. 



Invertebrate hosts : The commonest alternative hosts, 

 either intermediate or definitive, are insects, but some of 

 the ticks, ixodinae and argasinae, may also serve as hosts. 



Insects are infected with animal parasites in various 

 ways : 



(1) The blood-sucking insects draw up with the 

 blood any small parasites present in that fluid, such as 

 the parasites of malaria. These parasites developing and 

 multiplying in the insect host are ultimately injected into 

 a warm-blooded host man in this instance and multiply 

 again in that host. 



(2) In other cases, as in the development of piro- 

 plasmata and the spirochaeta in ticks, the development 

 of the parasites taken up with the blood is continued in 

 the eggs of the host, and the full development does not 

 take place till the eggs are hatched and the young ticks 

 are sufficiently developed to bite a warm-blooded host, 

 when they will transmit the infection. 



(3) The parasites drawn up with the blood may 

 develop in the lumen or walls of the alimentary canal 

 and the sporozoites may be discharged with the faeces. 



(4) The larvae of the invertebrate host living in water 

 become infected directly through their food with protozoa. 

 These then develop and, as in the case of a monocystis 

 described by Ross, multiply after encystment, so that 

 when the insect has attained its perfect form the 

 imago it harbours very numerous parasites which are 

 deposited with the excrement and then live an indepen- 

 dent existence till they re-enter other larvae. It is 

 possible that many of the flagellata are thus transmitted. 



Protozoa are not the only parasites for which inverte- 

 brates act as hosts. Several of the metazoa are conveyed 

 in a similar manner. Bacteria also can be conveyed by 

 such hosts. In some instances the insects merely act 



