We can scarcely turn one of these flat stones which lie half-buried
in sandy mud at the water’s edge without finding one or more of this
tribe. Let us try. Here at once is a specimen, one of the finest as
well as one of the commonest of all.[83] The upper surface is of a warm
fawn-brown; but the beautiful flashes of iridescent blue that play on
it in the changing light, and the exquisite pearly opalescence of the
delicate pink beneath, are so conspicuous as to have secured it the
title of “Pearly,” _par éminence_. As you gaze upon it you see the
great dorsal blood-vessel or heart, as a dark red line running along
the middle of the back. This, at irregular intervals of five to fifteen
seconds, contracts almost to invisibility, and then expands again,
taking the form of a string of beads in each process.
This, however, is only a part of the great circulating system in this
fine worm. In it the red blood flows from the tail towards the head; at
the under side of the body, there is a similar vessel (the ventral),
through which the blood flows from the head towards the tail. Besides
these, there are four other longitudinal vessels in the interior of
the body, one above and one below the intestine, and one on each
side, all of which are connected by transverse branches. Thus there
is a double circulation. “The great dorsal,” says Dr. Williams, “the
reservoir of the centripetal streams of the body, may be likened to a
right ventricle (the lungs cut off), and the great ventral to a left
ventricle. The duty of the former is to collect the refluent blood of
the system; of the latter to circulate it again.”
Our _Nereis_ is furnished with two pairs of dark blue eyes, and with
an evertile proboscis, which consists of two segments. The one first
protruded is beset on its upper side with some largish horny points,
arranged in two groups, which are continued round to the under side in
a band of irregular lines of points. The second segment has four groups
of points, the groups of oblong outline, radiating from the strong
black jaws, which are protruded last, and can be widely opened and
closed.
Here, too, we have an Annelid of very different form and
appearance.[84] It is a mean-looking worm, about an inch and a half
long, of flattened shape, blunt at each end, apparently covered with a
smooth skin of a dull brown colour; but on being touched it presently
throws itself into elegant serpentine curves, and then what appears to
be the upper skin is seen to be composed of a great number of round
flat membranous plates or shields, arranged in two rows overlapping
each other. These, though of large size, are attached to the body only
by a small point in the centre of their under side, so that when the
animal moves, the edges of these shields are lifted, and reveal their
true structure, sliding upon one another in a singular manner.
In captivity I have found the animal inert, prone to seek concealment
in the corners of the glass vessel beneath other animals or stones,
and remaining still for a long time together; but, if disturbed,
manifesting impatience and much agility, swimming through the water
with strong lateral undulation, as already described. I have been
struck with the deciduous character of the broad leaf-like shields.
As soon as the animal was in my possession, one and another of these
appendages was cast off at almost every time that I roused it to
action; and even when allowed to remain quiet, the denuding process
went on, so that in a few days it was nearly half naked. This process
of self-mutilation will sometimes go on till scarcely a single shield
remains out of the double series.