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downward and backward; in this way a progressive movement in a straight line is produced.
Alterations in the direction of the movement are broiight about chiefly by modification in the
force of the strokes of one or more of these pairs of limbs.
If we combine these facts with those put forward on pp. 117 and 118 above, we obtain
the following results:
I. In all planktonic Ostracods whose natatory limbs give the body a progressive
movement in a straight line the shell has no processes that effect the direction of the
motion. Such forms can be divided in to two categories:
1) Those whose shells are quite without any large processes (Cypridinids, most
of the Halocyprids* * and all Cypri d s).
2) Those whose shells have processes. These processes, however, either point in the
direction of the movement (the longitudinal axis of the body) or are developed as lateral,
symmetrically situated, wing-like formations (a number of Halocyprids).
II. In the only planktonic Ostracod (Thaumatocypris echinata) whose limbs do
not give the body a progressive motion in a straight line the shell has processes that effect
the direction of the motion by their position.
With regard to the processes that are found in the representatives of category I:
2 it seems to be beyond doubt that they function as buoyaney organs, as they all contribute
more or less to increase the résistance of projection. But this does not seem to be the only
funtion, perhaps it is not even the most important one. That this is the case seems to be
shown partly by the faet that many of them do not have their maximum extension in the
horizontal plane and partly because the forms in which they are developed are very strong
swimmers. It is certain that they also function as stability organs. The way in which the
posterior part of the shell and the rostrum point in Conchoecia daphnoides — see fig. XVI
above — resembles as a matter of faet very mueh the arrangement of the metal plates that
we see on submarines, plates that do not increase the buoyancv power in these vessels, but
are designed to increase the stability of the motion (besides influeneing its direction). A study
of the shape of the shell in this species will show that it very closely approaches the ideal of
a swift and stable swimming organism.
It thus remains to analyse the function of the spines on the shell of Thaumatocy pr is
echinata. In this species, as is seen above, the mechanica! arrangements for swimming resemble
rather closely those of the genus Bosmina. The body is preåsed forward and upward by
the natatory strokes of the first and second antennae and for the same reasons as in the
last-mentioned species a continuai backward rolling movement would be produced if there
were no special organs to prevent this. As in Thaumatocy pr is the natatory limbs are not,
times in the neighbourhood of the bottom, sometimes higher up, sometimes right up to the surface of the water,
about two decimetres from the bottom. In other words it acted in the aquarium in about the same way as most
Cypridinids. A doser investigation showed that in swimming the first antennae struck upward and backward
and somewhat outward, while the exopodite and endopodite of the second antenna and maxilla struck downward and
backward and somewhat outward. Alterations in the direction of the motion were produced chiefly by modification
of the force of the strokes in one or more of these limbs.
* To these belong the Polycopids too.
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