Serpulæ and Sabellæ.
[Illustration:
1, 2 SEA-URCHIN'S TEETH (Two illustrations.)
3 COMMON SEA CUCUMBER
4 COMMON RAZOR-SHELL
5 COMMON SERPULÆ, attached to a piece of stone.]
XXII.
With the exception of the Balani (Acorn-Barnacles), perhaps the most
common objects to be met with at the sea-shore are the Serpulæ.
Scarcely a rock, or shell, or bit of old china, or piece of wood, or
rusty nail, lying near low-water mark, but is encrusted with colonies
of these animals. I have a small twig of a tree by me, so thickly
coated with Serpulæ as to obscure all signs of its ligneous character,
except at each end. A shell also exhibits the same phenomenon, and
well-nigh defies the most skilful observer to define its original form
with any degree of certainty.
The shelly tubes of these animals are built in the form of serpents,
or twisted funnels, of a milk-white colour. Although so extremely
hard, these tubes are formed solely by an exudation from the body
of the animal--a simple marine worm. Unlike its erratic friend, the
earth-worm, the Serpula is sedentary in its habits, and at no time does
it ever leave its dwelling.
The delicate, but brilliant feathery plume--the only portion of the
animal ever visible--constitutes the principal mechanism by means of
which the Serpula constructs its calcareous tube.
A most wonderful instance of how mighty are the works which these
insignificant creatures form when congregated together in vast numbers,
and how useful such labours may sometimes be to mankind, is narrated by
Dr. Darwin in his 'Voyage of the Beagle.'
Being delayed by adverse winds, this gentleman made a stay at
Pernambuco, a large city on the coast of Brazil, and the most curious
object that he saw there was the reef that formed the harbour. 'I
doubt,' to use his own words, 'whether in the whole world any other
natural structure has so artificial an appearance. It runs for a length
of several miles in an absolutely straight line, and parallel to, and
not far distant from the shore. It varies in width from thirty to
sixty yards, and its surface is level and smooth; it is composed of
obscurely stratified hard sandstone. At high water the waves break over
it; at low water its summit is left dry, and it might then be mistaken
for a breakwater erected by Cyclopean workmen. On this coast the
currents of the sea tend to throw up in front of the land long spits
and bars of loose sand, and on one of these the town of Pernambuco
stands. In former times a long spit of this nature seems to have become
consolidated by the percolation of calcareous matter, and afterwards
to have been gradually upheaved, the outer and loose parts during the
process having been worn away by the action of the sea, and the solid
nucleus left as we now see it. Although night and day the waves of
the open Atlantic, turbid with sediment, are driven against the steep
outside edges of this wall of stone, yet the oldest pilots know of no
tradition of any change in its appearance. This durability is by far
the most curious fact in its history; _it is due to a tough layer, a
few inches thick, of calcareous matter, wholly formed by the successive
growth and death of the small shells of Serpulæ, together with some
few Barnacles_, &c. These insignificant organic beings, especially
the Serpulæ, have done good service to the people of Pernambuco, for
without their protective aid the bar of sandstone would inevitably have
been long ago worn away, and without the bar there would have been no
harbour,'
Nothing whatever appears to be known relative to the mode of
reproduction of these Annelids. I have paid much attention to the
subject, but as yet have not gained any positive information regarding
it. The only fact which I consider worthy of being chronicled is the
following: On one occasion, when quite a novice in Marine Zoology,
while observing a beautiful group of Serpulæ seated on a stone, I saw
issuing from out one of the tubes a kind of very fine dust, of a rich
crimson hue, which continued to arise for nearly an hour in spite of
repeated efforts to disperse it by aid of a camel-hair pencil. At first
I believed the 'dust' to be the 'remains' of a deceased serpula, but
afterwards found that such was not the case, the annelid being alive
and healthy. Never having seen the phenomenon since, it has been a
great source of regret to me that I did not endeavour to discover what
the dust was composed of; but have little doubt that the microscope
would have shown it to be, in reality, the ova of the Serpula.
Another class of Annelidans, termed Sabellæ, like the Serpulæ, also
build habitations for themselves, but not of the same materials.
Instead of being white, the tubes of the first mentioned animals are
brown in colour, and composed of minute granules of sand, or small
shells, and lined internally with a gelatinous substance exuded from
the body of the worm. On the interior of the oyster and other shells,
and even in univalves occupied by the Lobster Crab, various tubes of
Sabellæ may often be seen. They are, however, generally discovered
congregated together, forming a kind of honeycomb mass in the fissures
of rocks, or against the sides of rock-pools, or on the surface of
small stones, &c.
A mass of Sabellæ tubes forms by no means an inappropriate or
unpleasant object for the tank, as the animals are hardy, and will
live for many months if the water be kept pure. Moreover, while in
confinement, they do not live in luxurious indolence, but ever seem
to be busy in the exercise of their architectural propensities, making
alterations, repairing damages, or otherwise 'sorting' their tubiculous
habitations.
'The tubes of the Sabellæ,' says Dr. Williams, 'are soft, flexible,
and muddy. Slimy mucus furnished by the integumentary glands of the
body is the mortar or cement, fine sand molecules are the "stones" or
solid material of the architecture. In the Sabellæ the lime of which
the tubes are built is held in solution in the mucus provided by the
cutaneous glands. It is adjusted in the fluid form, and moulded by
appropriate tools into the required shape. It then _solidifies, too,
under water_, like the "Aberthaw lime." The tube of the Sabellæ fits
closely round the body of the worm; it is slightly elastic, and the
interior is smooth.'