1909.] BLOOD-PARASITES OF FRESHWATER FISHES. 7 
by the expression “ pelotonné.” The object of these movements 
is probably simply to increase the animal’s power of absorbing 
nutriment &e. from the surrounding medium, by incessantly 
changing the surface of contact between the body and the blood- 
plasma, just as a Z7’whifex in the mud wriggles incessantly in order 
to bring the surface of the body into contact with fresh water and 
oxygen. I think it highly probable that when these parasites are 
being carried round passively in the circulation they perform 
wriggling movements only, having no occasion to travel. On the 
slide, however, they are seen sometimes to travel, sometimes to 
wriggle ; but when they become moribund they only wriggle. 
T'rypanoplasma, so far as I have observed, travels always with 
the anterior free flagellum forwards ; [have never seen it go with 
the posterior flagellum, that is to say the flagellum which runs 
along the edge of the undulating membrane, directed forwards. 
If the kinetonuclear extremities of 7rypanosoma and Trypano- 
plasma are to be considered homologous, then the direction in 
which the former usually travels is the opposite to that im which 
the latter invariably progresses. I have not observed in 7rypa- 
noplasma anything comparable to the wriggling movements o! 
Trypanosoma ; the former genus has the body relatively much 
shorter and less flexible. It can be observed frequently, however, 
that a Trypanoplasma when travelling in a certain direction will 
quite suddenly bend over on itself and travel in a direction more 
‘or less the opposite to that which it took formerly. 
A vemarkable feature of some trypanosomes and trypanoplasms 
of fish is the great disparity in size between different individuals 
in the same blood. This point has already been noticed by previous 
observers with regard to the trypanosomes of the pike and the 
-eel, both of which have been divided, each into two varieties, distin- 
guished as var. parva and var. magna respectively. In the five 
fish-trypanosomes examined by me, I find the state of things 
different in different species. In the trypanosomes of the bream 
-and the tench, which are perhaps one and the same species (figs. 
27-32), I could not find any variation in size sufficiently well 
marked to be characterized as true dimorphism; but my material 
-of these two forms is not so abundant as in other cases. In the 
trypanosome of the pike (7'. remaki, figs. 20-26) I found the well- 
marked dimorphism described by Laveran and Mesnil, and though 
the two forms parva and magna each vary slightly in size and 
other characters, they were nevertheless easily distinguished, and 
no forms could be found transitional between them. In the try- 
panosome of the perch (7. perce, figs. 8-14) I found three principal 
types—small slender forms, large stout forms, and intermediate 
forms. The small forms and the intermediate forms are connected 
by transitions (figs. 10, 11), but the stout forms stand rather apart, 
owing to the shortness of the free flagellum (figs. 13, 14), Finally in 
the eel, I found every possible gradation between the smallest and 
the largest forms (figs. 1-7); there is a very great difference in 
size between the two extremes, but the absence of a dividing line 
