t it stick out a part of its body: a vacuum
is formed behind this sort of piston, which may be compared with that of
a pump. Thanks to the rear window, a valve without a plug, this vacuum
at once fills, thus renewing the aerated water around the gills, a soft
fleece of hairs distributed over the back and belly.

The piston stroke affects only the work of breathing; it does not alter
the density, makes hardly any change in that which is heavier than
water. To lighten the weight, the caddis worm must first rise to the
surface. With this object, it scales the grasses of one support after
the other; it clambers up, sticking to its purpose in spite of the
drawback of its faggot dragging through the tangle. When it has reached
the goal, it lifts the rear end a little above the water and gives a
stroke of the piston. The vacuum thus obtained fills with air. That is
enough: skiff and boatman are in a position to float. The now useless
support of the grasses is abandoned. The time has come for evolutions on
the surface, in the glad sunlight.

The caddis worm possesses no great talent as a navigator. To turn round,
to tack about, to shift its place slightly by a backward movement is all
that it can do; and even that it does very clumsily. The front part
of the body, sticking out of the case, acts as a rudder. Three or four
times over, it rises abruptly, bends, comes down again and strikes the
water. These paddle strokes, repeated at intervals, carry the unskilled
oarsman to fresh latitudes. It becomes a voyage on the right seas when
the crossing measures a hand's breadth.

However, tacking on the surface of the water affords the caddis worm
no pleasure. It prefers to twitter in one spot, to remain stationary in
flotillas. When the time comes to return to the quiet of the mud bed
at the bottom, the animal, having had enough of the sun, draws itself
wholly into its sheath again and, with a piston stroke, expels the air
from the back room. The normal density is restored and it sinks slowly
to the bottom.

We see, therefore, that the caddis worm has not to trouble about
hydrostatics when building its scabbard. In spite of the incongruity of
its work, in which the bulky and less dense portions seem to balance
the more solid, concentrated part, it is not called upon to contrive
an equipoise between the light and the heavy. It has other artifices
whereby to rise to the surface, to float and to dive down again. The
ascent is made by