24. REPORTS OF THE DEPARTMENT OF COMMERCE. 
the use of all available means of protecting and increasing the supply 
of top minnows (Gambusia) in the area under protection, and the 
careful observation of the effectiveness of these and other fishes in 
the extermination of mosquito larve. To increase the number of 
minnows in the extra-cantonment area, propagation was resorted to 
and fish were also brought in from places outside of the protected 
area. As the top minnows were found to be a favorite live bait in 
angling, the cooperation of the public was sought and received 
through the use of posters, placed at or near the various small ponds, 
bearing a warning that the small fish were given protection as a 
health measure. 
It has been fully demonstrated that the small fishes are in many 
cases most effective agents for the control of mosquitoes, but it has 
also been positively ascertained that the efficiency of fish even when 
present in abundance is by no means universal and complete. Much 
depends upon the physical and biological conditions in the water, 
such as the presence of débris and of plants of various species, wave 
action, fluctuations of level, and various other factors. There is, as 
yet, lacking the degree of knowledge necessary to define fully the 
conditions under which fish are effective, or to govern the change 
of conditions so as to make the fish as efficient as is desired. It is 
with reference to securing a sure foundation of knowledge concerning 
the relations of fish and mosquito larve that the campaign of obser- 
vation and experiment in cooperation with the Bureau of Entomol- 
ogy was undertaken two years ago at Mound, La., as mentioned in 
previous reports. The investigations at Mound have been continued 
actively and bid fair to make substantial contributions to knowledge 
which will be of direct and decisive importance in the future conduct 
of antimalarial compaigns. 
‘DISEASES AND PARASITES OF FISHES. 
The Bureau has given special attention to the diseases of fishes as 
bearing upon the loss of fish in hatcheries, fish ponds, and public 
waters. While the subject is broad and the problems arising are 
difficult and tedious of solution, substantial progress has, neverthe- 
less, been made during the year in the study of some of the affections 
to which fish are subjected in artificial and natural bodies of water. 
A new parasite of the buffalofish—An investigator of the Fish- 
erres Biological Station at Fairport, Iowa, has discovered a new and 
evidently important form of trematode worm which infests buffalo- 
fish in ponds. The life history of the parasite has been definitely 
worked out and is briefly as follows: | 
The adult trematode, living in the alimentary tract of the buffalo- 
fish, expels its eggs, which pass out into the water. From each of 
these eggs, which are “laid” during the late summer, there hatches 
in the fall a ciliated embryo (mericidium), which swims in the water 
until it finds a snail of the species Planorbis trivolwis. Into this 
snail it works its way and eventually encysts in its host’s liver. In 
the usual manner it grows and multiplies there during the winter and 
spring, and in summer emerges from the snail as a tailed and styleted 
cercaria. This larva can live for several hours in the water, but 
must find a May fly larva for its second intermediate host, into which 
