oo 
family Gamasida has acted as the intermediate host for a protozoan 
parasite. 
So far as accurate knowledge extends, when a parasitic protozoon 
recpiires two hosts for the completion of its hfe cycle one of these hosts 
will be found to belong to the phylum Arthropoda. An apparent 
exception exists in Hsemogregarina stepanr/vi in the tmtle {Emys 
orhicularis) , the invertebrate host of which is a leech (PlacohddJa 
catemgera).^ 
Of the live subdivisions or classes into which the phylum is divided, 
two — the Insecta and the Arachnida — ^include the species wliich act as 
iutermediate hosts. 
Of the insects which serve as parasitic hosts and agents of trans- 
mission for pathogenic protozoa the mosquito is the best known, and 
the malarial parasite the only form which has been accimately worked 
out in all stages. 
The life cycle of malaria in birds (Proteosoma grass-ii) is almost as 
well worked out as malaria in man. Quite recently Christophers has 
described the complete sexual cycle of Piroplas-ma cams in the dog 
tick. Pi. sanguineus. 
Of insects^ a nimiher, mcluding biting flies, lice, bedbugs, and fleas, 
are known to be concerned in the transmission of blood parasites. 
Some of these have been proved to be the agents of transmission, but 
whether as true intermechate hosts or merely carriers is as yet imde- 
termined. in many instances, the sexual cycle being unknown. 
In the class Araclmida the ticks have constituted, up to the present 
time, the only group capable of transmitting protozoan disease. The 
mmiber of known tick-home chseases is increasing and they are of 
great importance. In an epoch-marking work upon Texas cattle 
fever Smith and ICilbome.^ in 1S93. were the first to show the part 
played by the Arthropoda (in this instance ticks ' in conveyhig dis- 
ease. Various forms of piroplasmosis. relapsmg fever, Rockr Moim- 
tain spotted fever, and Leucocytozoon cards have been shown to be 
transmissible by ticks. 
Until quite recently it was beheved that ticks transmit mfection 
only tlmough their progeny : i. e.. only ticks derived from the eggs 
of infected mothers have power to inoculate susceptible animals. 
Recently it has been shown that infection with certain kinds of 
phoplasma is taken during the nymphal stage, but is transmitted only 
by the adult tick. 
« See p. 9. ref. 
& Smith. Th.. X Kilbome: Texas fever. Bull. Xo. 1, Bur. Animal Indus.. U. S. 
Dept. Agric.. Wash., 1S93. 
