d portion, the little teeth at the head, the arms, etc., can
be interpreted in the most beautiful way, above all the form of the
anchor itself, for the two arms prevent it from swaying round to the
side. The position of the anchors, too, is definite and significant;
they lie obliquely to the longitudinal axis of the animal, and
therefore they act alike whether the animal is creeping backwards or
forwards. Moreover, the tips would pierce through the skin if the
anchors lay in the longitudinal direction. Synapta burrows in the
sand; it first pushes in the thin anterior end, and thickens this
again, thus enlarging the hole, then the anterior tentacles displace
more sand, the body is worked in a little farther, and the process
begins anew. In the first act the anchors are passive, but they begin
to take an active share in the forward movement when the body is
contracted again. Frequently the animal retains only the posterior end
buried in the sand, and then the anchors keep it in position, and make
rapid withdrawal possible.

Thus we have in these apparently random forms of the calcereous
bodies, complex adaptations in which every little detail as to
direction, curve, and pointing is exactly determined. That they have
selection-value in their present perfected form is beyond all doubt,
since the animals are enabled by means of them to bore rapidly into
the ground and so to escape from enemies. We do not know what the
initial stages were, but we cannot doubt that the little improvements,
which occurred as variations of the originally simple slimy bodies of
the Holothurians, were preserved because they already possessed
selection-value for the Synaptidae. For such minute microscopic
structures whose form is so delicately adapted to the role they have
to play in the life of the animal, cannot have arisen suddenly and as
a whole, and every new variation of the anchor, that is, in the
direction of the development of the two arms, and every curving of the
shaft which prevented the tips from projecting at the wrong time, in
short, every little adaptation in the modelling of the anchor must
have possessed selection-value. And that such minute changes of form
fall within the sphere of fluctuating variations, that is to say,
_that they occur_ is beyond all doubt.

In many of the Synaptidae the anchors are replaced by calcareous rods
bent in the form of an S, which are said to act in the same way.
Others, such as those of the genus