It is of a pentagonal figure, with the margin a little receding
between the angles, but not so as to cause the latter to form distinct
arms. The body is flat below, and plump and cushion-like above, of a
yellowish olive hue, with the very edge of a golden orange tint, while
a spot of the latter colour, a little out of the centre, marks the
situation of a remarkable organ called the madreporiform tubercle, the
proper use of which has not as yet been satisfactorily determined. Just
at the extremity of each angle, but a little below, is situated a wart
of rich crimson hue, which is supposed to be an eye, being seated on a
small ganglionic swelling of a nerve that passes along the ray. It is
true no crystalline lens has as yet been detected on the pigment dot,
either in this species or any other (for the specks are found in the
same position in all the proper Star-fishes), but they are manifestly
of the same character as similar specks in _Rotifera_, and other humble
animal forms, which in some examples are connected with an indubitable
lens. It might seem at first as if the situation of these eyes were
not very favourable to vision; but, in truth, they command the ground
just before and around the ray-tip, and also the water in a horizontal
direction; and as there is one at each of the five angles, the entire
circumferent space is viewed at each moment. Add to this, that the
animal is in the habit of very frequently turning up the tip of one or
other of its rays, when the range of vision would take in the zenith;
and we shall perceive that no position in any other part of the body
could be so suitable for the location of eyes as these selected. It is
not to be supposed that distinct definition of objects is attained by
these rudimentary organs; but the animal is probably conscious of the
difference between light and darkness, and may also discern the sudden
approach of any object, either by its interception of light, or by its
colours, though its features and form may be indistinct. Such a degree
of visual perception, though very imperfect in our esteem, may be of
great use to this sluggish creature, and amply sufficient for its need.
The whole upper surface of this Starlet is covered with six-sided
convex plates, each of which is crowned by a group of short blunt
spines; the number in each group varying from one to nine; four to
six, however, most commonly; arranged in a diverging tuft. Over the
madreporiform tubercle, the tufts are stouter, and bend towards
each other, as if to protect this delicate organ, which, whatever
its function, is grooved with sinuous furrows, visible only with a
considerable magnifying power, like the rounded masses of coral from
the tropical seas, called brain-stones.
On the under surface similar stout short spines are arranged in
transverse bands across each ray, interrupted, however, by the central
furrow, which is perforated with two ranges of orifices to give issue
to the sucker-feet, which thus form a double row. These organs do not
differ importantly either in structure, function, or appearance, from
those in the Cross-fishes, in the Sea-cucumbers, or in the Urchins.
In health and activity the whole upper surface is covered with a forest
of short pellucid tubes, closed at the tips, which protrude from pores
in the plates, and stand erect, moving, however, to and fro at the will
of the animal; they are evidently filled with fluid. I cannot find any
notice of these organs: they are probably connected with respiration.