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The first-mentioned writer has in the case of these forms accordingly written the
above-quoted buoyancy formida established by W. OSTWALD, in the following way:
_. , , . . Übergewicht x Abwärtsbewegung und -Steuerung
„Sinkgeschwindigkeit = —;—
Reibung x Aufwärtsbewegung und -Steuerung“.
The factor „Reibung“ in this formula, which includes both the viscosity of the water
and the friction between the sinking body and the water, i. e. the form-resistance, appears,
according to this author, in these forms (as in the actively swimming forms of Crustacea in
general) not to have „eine besonders große Bedeutung“, p. 480.
The shape of the body in the pelagian Cladocera varies, however, to a very great extent;
many of them are characterized, for instance, by more or less excessivelv developed processes
of different kinds.
How are these processes to be explained?
The most widely spread view seems to be that these processes are to be explained
as buoyancy organs.
A view that differs somewhat from this is put forward by C. Wesenberg-LUXD;
in a work of 1908 this eminent author writes as follows (p. 12): „I am inclined to
believe on the whole, that many of these buoyancy-organs which have hitherto been
considered of importance only in as far as they increase the cross-section résistance
and surface-area, play a by no means small role in shifting the centre of gravity of the
body“, i. e. a number of these processes function as balance - organs as well as buoyancy
organs. In support of this view observations made on the genus Bythotrephes are given
in the work mentioned; thus we read (p. 12): „As mentioned above I have never seen
Bythotrephes floating; it hops about always in the aquaria with innumerable, small and
short jumping movements; it gives one the impression of being an excellent swimmer but
not a floating organism. I have never seen it use its long, posterior legs as outriggers; it drags
the long spine behind it during swimming and we might think on a cursory glance that this
was rather a hindrance. The direction of movement is as a ride distinctly horizontal. If we
now remove the spine, which can easily be done with a good pair of scissors, we change the
Bythotrephes to a dancing figure, waltzing round and round in spirals or closed circles; they
finally end at the bottom, from which they never again rise. The spine has thus actually
been a balancing organ, which has played the rôle of moving the centre of gravity, so
that a horizontal movement could be possible; further it is a buoyancy organ, which
according to its point of insertion and the position it gives the body in the water augments
the cross-section résistance.“
The same view (which was afterwards adopted by F. E. RÜHE, 1912, among other writers)
had already been previouslv (1896) expressed by C. Cili x with regard to a number of other
pelagian Crustacea. After an expression of this opinion we read in the work mentioned (p. U*3):
„Sind die betreffenden Formen mit kräftigen Ruderfüßen versehen, so liegen die Balancirstangen
horizontal in der Mediane und bedingen bei dem Durchschneiden des Wassers eine geradlinige
Fortbewegung und einen geringen Widerstand,“
How are the more or
less extensive
processes on the shell oj
the Cladocera to be
explained?
1) Buoyancy organs.
2) Balance organs.
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