W. S. Patton 
93 
have seen rats (Mus decumanus) succumb to an infection of T. lewisi and 
Jurgens (1902), Francis (1903) and others have made similar observa¬ 
tions. The subject however is complicated by the production of active 
immunity against trypanosomiasis, and it is not therefore possible with 
our present limited state of knowledge to draw any final conclusions from 
the facts already ascertained. 
Except in the case of T. equiperdum, trypanosomes are transmitted 
from one vertebrate host to another, through the intermediation of a 
blood-sucking invertebrate. Woodcock notes that the actual relations 
existing between any particular trypanosome and its transmitting 
invertebrate are at present uncertain, but that in most cases the inverte¬ 
brate acts as a true host. According to Woodcock the investigations of 
Ldger (1904), Brumpt (1906) and Keysselitz (1906) have made it clear 
that the trypanosomes of fishes undergo a definite evolutionary cycle in 
particular organs of leeches which have fed on the infected fish. The 
development however can only proceed to its fullest extent in a certain 
leech. For example, T. granulosum of the eel only undergoes its cycle of 
development in Hemiclepsis marginata ; in other leeches, such as Callob- 
della punctata, Hirudo troctina and Piscicola geometra, the parasites only 
undergo partial evolutionary changes and then die off. This restriction, 
Woodcock says, distinctly points to the leech in question ( Hemiclepsis) 
being the specific natural host of T. granulosum. It is true that Brumpt 
has succeeded in infecting healthy fish with trypanosomes by placing on 
them leeches already infected with the parasite; these experiments 
however by no means prove that this is the only method by which 
trypanosomes are transmitted from one fish to another. Thomson 
(1908), who has recently studied a trypanosome of gold fish, was unable 
to find a single leech on any of the fish from a particular pond, although 
a large number of fish were examined, the majority of which were 
infected. These observations clearly suggest that piscine trypanosomes 
may be transmitted by some method other than by the bites of 
leeches. 
According to Schaudinn’s (1904) researches T. noctuae undergoes its 
developmental cycle in Culex pipiens and there is a regular periodicity 
in the infectivity of the mosquito. Prowazek has also described the 
development of another vertebrate trypanosome T. lewisi in the rat 
louse Haematopinus spinulosus ; and considers this insect acts as a true 
alternate host of T. lewisi. We would draw attention to Nuttall’s (1908) 
recent transmission experiments with this parasite in which he clearly 
demonstrates that it is transmitted from rat to rat by means of Cera- 
