1909.] BLOOD-PARASITES OF FRESHWATER FISHES. 13 
note any of the peculiarities of the trypanosome of the perch, or 
to take into account its variations of size and structure. 
Trypanosomes were found more or less abundantly in almost all 
the perch examined by me. In a few of these fish none was seen, 
or they were found only after more prolonged searching ; as a 
rule, however, they were found at once and were present in con- 
siderable numbers. If a fish was not well infected, I did not 
trouble to make smears of it or to examine it further. Hence 
the few cases in which I did not find trypanosomes were probably 
cases in which careful search would have revealed the presence of 
the parasites in scanty numbers. I doubt if any of the perch 
were really entirely free from them. From the perch I examined, 
I gained the impression that the trypanosomes were rather more 
abundant in small or medium-sized fishes than in those of the 
largest size. I did not find trypanoplasms in any of the perch 
examined *, 
Examination of the blood freshly drawn from the perch showed 
that the trypanosomes differed considerably in size. They may 
be divided, speaking generally, into large and small forms. The 
large forms were much the most abundant; the small forms were 
very scarce. The large forms when seen living (figs. 94, 95) 
appeared stout and sluggish, as a rule of considerable size, but 
showing marked variations in this respect, some being smaller 
and more active. They wriggle incessantly but do not travel 
much. The body is spindle-shaped; one end, which bears the 
flagellum, is greatly attenuated and sharply pointed ; the other 
is also drawn out, but is much less attenuated and appears to end 
more bluntly. At the blunt end a distinct, very refringent dot 
could always be seen, doubtless representing the kinetonucleus ; 
it appeared light at a high focus, black at a low focus. Some- 
times two small dots could be seen (fig. 94), in which case the 
second was probably the blepharoplast. The nucieus could be 
seen distinctly as a rounded clear space in which the karyosome 
appeared as a darker spot, not very refringent. A short way 
behind the nucleus there was seen in some specimens a distinct 
dot or grain, much more refringent than the karyosome, and 
apparently lodged in a clear space or vacuole (fig. 95); this body 
was not always seen. The undulating membrane and flagellum 
could be clearly distinguished, especially when the parasites were 
moribund and becoming slower in their movements, after being 
under observation for some hours. 
The trypanosomes show a great tendency in the living state to 
twist and roll themselves up, in a way that calls to mind a snake 
of stout build, such as a python. Their movements were of two 
kinds, which have been distinguished above as wriggling and 
travelling movements respectively. When wriggling they simply 
twist over and over, throwing themselves into $-like curves, 
without changing their place. When travelling the body is 
* Keysselitz (Arch. Protist. vii. 1906, pp. 2, 3) records the occurrence of both 
‘trypanosomes and trypanoplasms in Perca fluviatilis. 
