As the last of a course of lectures upon "Recent Scientific Researches
in Australasia," Dr. R. Von Ledenfeld lately delivered a lecture at the
Royal Institution, upon "Recent Additions to our Knowledge of Sponges."
The lecturer did not confine himself to the sponges of Australia alone,
but gave a _resume_ of the results of recent investigations on sponges,
together with several new interesting details observed more especially
in studying the growth of Australian sponges. With a passing reference
to some peculiarities of the lower marine animals of the Australian
coast, Dr. Ledenfeld remarked upon the preponderance of sponges over
other forms of marine life in that part of the world. It has long been a
point of discussion as to whether sponges belong to the vegetable or
animal kingdom, but naturalists are now generally agreed in regarding
them as animals, a conclusion, the lecturer remarked, that Aristotle had
also arrived at.
Sponges grow in a variety of more or less irregular shapes, but it has
been observed that the most regular structures occur in the calcareous
species. As to color, Dr. Ledenfeld remarked that some of the Australian
sponges are of exceptionally brilliant hues, while others range from the
black of the common sponge _(Euspongia officinalis)_ to a pure white.
Also, it may be remarked, the sponges growing in deep water are of less
decided color and more elastic in character than those living in shallow
water, and from the last named quality are more valuable in commerce.
The irregular honeycombed appearance of the sponge is due to a most
complicated canal system, consisting of a series of chambers through
which the water is drawn by the animal in always the same direction.
The inhalent pores are very minute, and open into small subdermal
cavities which communicate by means of interradial tubes with the
ciliated chambers, the latter being very small ramifications of the
interradial channels, and in them the movement causing the current of
water is maintained. From hence all faecal and other matter is
discharged through the oscula, the larger openings observed on the
surface of the sponge. Dr. Ledenfeld showed the different parts of
sponges by means of microscopic slides thrown on to a screen, and also
the shape and arrangement of the chambers in different species. The
ciliated chambers especially attracted attention. They are very small
and circular, and the interior is clothed with cells very similar to the
cilia cells in higher animal life.
These cells are arranged around the ciliated chambers in the form of a
collar, and from each cell flagella protrude, which are in continual
motion. These flagella, like bats' wings, are capable of being bent in
only one direction, so that, in the course of their pendulum-like
motion, in the movement one way the flagella are bent, while in the
return movement they remain stiff, thus causing a current of water
always flowing in one and the same direction. These ciliated chambers
are easily detected in the sponge by means of a microscope, as they
appear more highly colored. After the lecturer had thus given a general
outline of the structure of the sponge, he drew attention to the
character of its food and its method of digestion. It is not known
exactly what the sponge lives upon, but if upon other animals they must
be necessarily very small, owing to the size of its inhalent pores.
The sponge, like the tape-worm, has no stomach, but must absorb its food
through the outer skin from matter in a soluble state, similarly to the
roots of trees. This process of absorption is probably accomplished in
the interradial or ciliated chambers, more probably in the former, as
the latter are generally considered excretory in function. Lime or
silica must also be absorbed from the water by most sponges in order to
make up the skeleton. The skeleton of calcareous sponges consists of a
number of spicules composed of carbonate of lime. These spicules are of
very varied though regular shape, but ordinarily assume a rod-like
needle shape or else a stellate form. In silicious sponges the spicules
are composed of silica, and are generally deposited around axial rods in
concentric layers. The spicules are joined together and cemented by a
body that has been named "spongin," which has much the same chemical
composition as silk, and, like silk, is very elastic. In some varieties
of sponges, especially in the kinds which come into the market, the
skeleton is almost entirely composed of fibers of pure "spongin." These
fibers are so close together as to draw up water by capillary action,
and, indeed, a great deal in the value of a sponge depends upon the
fineness and tenuity of these fibers.
Dr. Ledenfeld again illustrated this stage of his lecture by means of a
number of microscopic slides in which the variety of shape and size of
these spicules and "spongin" fibers were shown. The spicules are some
crutch-like, others spined or echinated, while the deep-sea sponges
appear to grow long thick spicules, which attach the sponge to the
ground by means of grapnel-like ends. In some cases the skeleton seems
to be more or less replaced by sand, the small grains of which are
cemented together by the "spongin."
Dr. Ledenfeld then drew attention to the presence of more highly
developed organs in the sponge. Muscles pervade the whole tissue of the
sponge, but are found more particularly in the superficial parts. One
set of muscles affect the size of the inhalent pores, causing them to
contract or expand, while another set are able to close the pores
altogether, thus acting as a protection from the attack of an enemy. All
these muscles are composed of spindle shaped cells, which are capable of
spasmodic motion, but recently in an Australian sponge, the _Euspongia
canalicula_, the lecturer said he had observed muscles approaching very
nearly in character those of the human frame.
That sponges have nerves is a discovery of recent date by a member of
the Royal Microscopical Society. Dr. Ledenfeld also about the same time
found indications of the presence of a nervous system, but the form in
which he observed the nerves at first apparently differed from those
observed simultaneously. This difference, however, he afterward found to
be due to the manner in which the section had been prepared for
observation. The nerves consist of two cells at the base of a cone-like
projection on the epidermis, and from each cell a fiber runs to the
point of the cone, besides several others connecting them with the
interior of the sponge.
It is remarkable that here again Aristotle has predicted that sponges
have a nervous system, basing his statement on the fact that ancient
Greek mariners foretold storms by the alleged contraction of the sponge.
The reproductive organs of sponges are also very highly developed, and
both ova and spermatozoa are found throughout the sponge, though more
concentrated in the interior. The ova consist of spherical cells, while
the spermatozoa resemble an arrow-head in shape. It has not yet been
ascertained whether two sexes exist in sponges, or whether the ova and
spermatozoa are produced at different periods by the same sponge. When
the embryo has become partly developed, it detaches itself from the
parent sponge, and, issuing from the oscula, propels itself through the
water by means of a number of flagella.
Silicious spicules next appear in its structure, and it then attaches
itself to a rock and assumes its mature form. Sponges are most numerous
in the waters of the temperate and sub-tropical zones, and the
salt-water varieties are by far more numerous than the fresh water.
Thus, while there are not more than ten fresh-water species known, Dr.
Ledenfeld remarked that about one thousand species of salt-water sponges
had been recognized. Each species of the salt-water sponge is, however,
generally found only in limited areas, and very few, all of which
inhabit deep water, are cosmopolitan. This is the more remarkable as Dr.
Ledenfeld asserts that all the sponges inhabiting the rivers of
Australia are identical with the fresh-water sponges of Europe, and in
order to explain this fact he put forward a rather interesting theory.
He assumes that sponge life in rivers has been originally generated by
the introduction of a single, or at most two or three germs by means of
aquatic birds. The inbreeding consequent upon this paucity of sponge
life has produced a certain fixity of character in fresh-water sponges,
and is in direct opposition to the effects of hybridization in the
salt-water sponges, by which they have acquired the capacity of adapting
themselves to local circumstances.
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