PARASITES OF SIMULIUM LARVAE ol 
In a half-grown larva there are three distinct areas in the cardia 
(pl. 6, fig. 8). The anterior cells are small and closely crowded 
together. These secrete very little material. Posteriorly there 
are a number of cells apparently undergoing degeneration since 
they are much less regular in shape than the intermediate cells. 
None of these has any secretion attached to the exposed surface. 
Between these extremes is situated a series of five or six large 
cells which are very actively secreting a material of a chitinous 
nature. This, by a process described below, is drawn out as a 
film into the mesenteron where it functions as the peritrophic 
membrane. 
In young and half-grown larvae all the anterior cells are small 
and almost functionless, while in mature individuals they are 
often enlarged and very actively secreting the material of which 
the peritrophic membrane consists. This, however, is not always 
the case for in some mature larvae they remain small. They 
seem, therefore, to constitute a reserve in case more secretion 
is required, and this reserve is not always drawn upon. That 
there is a backward movement of the cells of the cardia is indi- 
cated by the fact that those cells farthest behind the secreting 
portion have every appearance of degeneration, and though they 
are now, as far as can be seen, non-functional, the nucleus is 
much enlarged, though showing signs of disintegration. These 
cells also have the appearance of having been pressed out of 
shape and in some cases degenerate portions of them appear to 
be sloughing away. Then again there is no peritrophic membrane 
in the embryo, and it is not improbable that at this stage all 
the later functioning cells are undeveloped and are crowded 
together anteriorly. Unfortunately my preparations of the ear- 
liest stages obtained do not show details with sufficient distinct- 
ness to confirm this supposition. 
That the substance of which the peritrophic membrane is com- 
posed is chitinous is proved by the fact that it can be boiled in 
strong caustic alkalies without dissolving. Vignon (’01) showed 
that the membrane is produced as a fluid which becomes coagu- 
lated soon after secretion. From my preparations it seems that 
the material remains plastic for some time after secretion, and 
