78 
/meet Flagellates, etc. 
fresh contents of the stomach there were numerous dead flagellates, the bodies of which were 
distorted, the outlines and shape being very irregular and the cytoplasm vacuolised. In a 
stained preparation of the stomach contents in some of the flagellates the nucleus was. still 
visible, in others it was diffused in the cytoplasm, and in some not visible at all. No 
flagellates were found in the blood of this fish, but smears of the spleen and liver contained 
several of them. This result was obviously due to carelessness on my part, as in this case the 
spleen and liver were separated from the intestine after the latter had already been removed 
from the fish. In this way the organs named were liable to be contaminated from the contents 
of the digestive tract. This is confirmed both by the fact that all the flagellates seen by me in 
the fresh contents of the fish-gut were already dead and had partly undergone digestion, and 
by the experiments adduced above in which the technique employed prevented such accidents 
from taking place. 
In general, not only no harmful effect was observed from feeding the 
sticklebacks on the infected guts of Nepa for periods from one day to a 
fortnight, but they seemed to thrive on the food, being all the time very active 
and readily taking the guts whenever they were offered to them. Not one of 
the fish died from this food, and I may even say that the fish fed in this way 
were in better condition than the control fish kept in a tank with weeds and 
left to find food for themselves in the natural way. 
These experiments show clearly that in the fish the flagellates are digested 
in the same way as any other food, and the fact that the flagellates remain 
alive for a considerable time only points to their vitality and power of 
resistance. This is not so surprising, taking into account that the flagellates 
are already adapted to life in the insect gut, the digestive juices of which do 
not differ materially from those in the higher animals. 
These experiments also show that as long as any part of the insect gut 
remains undigested, thus protecting the flagellates enclosed in it from the 
action of the gastric juices, the flagellates in this part may remain alive. The 
action of the juices on the parasites is also exhibited in the fact that, whereas 
in the normal insect gut the flagellates are very numerous, very few can be 
Table II. 
Experiments on feeding sticklebacks on H. jaculum from Nepa cinerea. 
Time from 
Condition of food in the 
No. of 
last meal 
digestive tract of the fish 
Parasites in the 
No. of 
Total No. 
insect guts 
to exami- 
(examined fresh) 
blood, peritoneal 
Experi- 
of insect 
in last 
nation. 
A 
C 
Flagellates 
fluid, liver and 
ment 
guts fed 
meal 
Hours 
Insect guts 
spleen of the fish 
16 
4 
2 
2 
Undigested 
Living 
Absent 
17 
4 
2 
3f 
Partly digested 
99 
99 
18 
3 
1 
5 
99 
Few living 
99 
19 
12 
1 
5 
9* 
None found 
99 
20 
9 
1 
5 
99 
Dead 
99 
21 
1 
1 
18 
Digested 
99 
99 
22 
2 
2 
18 
Partly digested 
Few living 
99 
23 
10 
2 
24 
Digested 
None found 
99 
24 
10 
2 
24 
99 
99 
99 
25 
2 
1 
48 
99 
99 
99 
