37 
increasing thickness, the size of the bladder consequently keeps 
pace with the growing epithelial sac. But no traces of such a 
mouldering can be found in the interior of the bladder; and as the 
chitine evidently can not be dissolved in the air contained in the 
bladder, no doubt we must also drop this hypothesis. 
3^- Humid chitine is somewhat elastic; it seems not impos- 
sible that the different elements might be somewhat widened during 
the growth of the epithelial sac. In adult larvæ both the dark 
and the bright stripes are broader than in young ones, especially 
in the dorsal and the ventral walls; towards the ends they become 
narrower and narrower, and it seems not unlikely that these nar¬ 
row stripes might widen and thus make the surface larger (Fig. 3 
A—E, B—F). 
4®- New stripes might be intercalated between the old ones. 
Very often we see either a dark or a bright stripe divide into two 
with a new stripe of the different kind originating between the 
branches, as the figures will clearly show (Fig. 3 D). This can very 
well be explicated by the faet that there are far more stripes on 
the convex than on the coneave sides; but it seems not impos- 
sible that their number also can inerease during growth. 
However, as it is impossible either to follow any of these ap- 
pearances with the microscope or to count all the stripes in a young 
and an adult larva respectively, we are unable to decide which of 
the two last suppositions is the correct one, or if they perhaps 
are equally correct, so that the air-bladder inereases in either way. 
Postscript. In the month of July 1915 when I made some 
observations in the lake Esrom in North Zealand with the appara- 
tus of Dr. Petersen for quantitative examination of the bottom fauna 
(vide Rep. from the Dan. Biol. Stat. XX, 1911), I always found the 
lake form of the Corethra larva abundant in the bottom proofs 
from the deeper parts of the lake (the proofs were taken in depths 
from 20 to 25 m) the number of the larvæ varying from 1500 to 
about 9000 per m^. In order to decide whether these larvæ were 
caught by the apparatus on its way down through the water, or 
if they really were living in the mud of the bottom, I let the ap¬ 
paratus go down to a depth of 1 m above the bottom where I set 
