54 E. H. STRICKLAND 
of digestion pass through it by osmosis. Since this is the case, 
and since, as before stated, the membrane is exceptionally thick 
and complete from the proventriculus to the anus, it would seem 
to be very difficult or even impossible for Microsporidian par- 
asites to gain access to the tissues of the larvae after the membrane 
has been formed. 
c. The relation between these structures and parasitism 
In describing the structure of the cardia I drew attention to 
the fact that there is no peritrophic membrane in the embryo, 
and it does not seem unreasonable to suppose that in the earliest 
larval stage the membrane does not entirely surround the food. 
Indeed I have reasons to believe that the first meal or so of the 
young larva is the most critical of its life if Microsporidian spores 
are present in the stream in which it hatches. These reasons 
are three in number. First, in all cases of parasitised individuals 
there are indications that the parasite gains admission to the 
body cavity at a very early period in the life of the larva, for 
I have never succeeded in finding a full or even half grown larva 
in which the early stages of the parasite were present. Secondly, 
owing to the minuteness of the spores, which are comparatively 
heavy and sink in water, and to the smoothness of their shells, 
they would readily pass through the fans of somewhat developed . 
larvae which have assumed a vertical position in feeding. In 
the earliest stages, however, as shown above, it is conjectured 
that food has to be sought out by the larva which picks up what 
it can find collected in small depressions in the stones over which 
it moves. In this way it is extremely likely that it would ingest 
one or more of the innumerable spores which have been liberated 
in the water and which have, whenever possible, sunk to the bed 
of the stream. The third reason is that, as inferred at the con- 
clusion of the last paragraph, some of the first food to escape 
from the proventriculus may not be entirely separated from con- 
tact with the mesenteron by the peritrophic membrane. For 
if one allow that at this time the membrane is already partially 
formed, it will not be present far beyond the orifice of the proven- 
