The natural history of sponges had, up to the middle of this century,
been comparatively neglected. Until 1856, when Lieberkuhn published
his treatise on sponges, very little or nothing had been written on
the subject. Later, Haeckel did much to determine their exact nature,
and it is now universally admitted that sponges form one of the
connecting links between the animal and the vegetable kingdom.
Sponges, generally considered, consist of fine porous tissue, covered,
during life, with viscid, semi-liquid protoplasm, and are held in
shape and strengthened by a more or less rigid skeleton, consisting
chiefly of lime or silica. The tissue consists of a very fine network
of threads, formed probably by gradual solidification of the threads
of protoplasm. The inorganic skeleton is formed by larger and smaller
crystals and crystalline threads. In the various families of sponges
the quantity of inorganic matter varies greatly; some sponges are
nearly devoid of an inorganic skeleton, while other families consist
chiefly of lime or silica, the organic tissue being only rudimentarily
developed.
As observed in their natural state, sponges are apparently lifeless.
When, however, a live sponge is placed in water containing some finely
powdered pigment in suspension, it will be noticed that in regular,
short intervals water is absorbed through the pores of the tissue and
ejected again through larger openings, which are called "osculæ."
Following up these into the interior, we find them divided into
numerous branches, the walls of which are, under the microscope, found
to be covered with minute cells, fastened at one end only and
oscillating continually. By means of these cells the sponge receives
its nourishment.
Sponges with very rigid inorganic skeletons may be divided into two
classes--calcareous and silicious--according to whether the skeleton
is chiefly composed of lime or silica.
Our engravings represent two species of the latter kind, which are, on
account of the peculiar appearance of their skeleton, called glass
sponges.
Fig. 1 represents the "sprinkling pot sponge," _Eucleptella
aspergillum_. It is generally found in very deep water throughout the
Pacific. Specimens were found over fifty years ago, but, as they had
to be brought up from depths between 500 and 800 fathoms, they
remained very scarce and sold at fabulous prices.
[Illustration: Fig. 1.--SPRINKLING POT SPONGE.--(_Eucleptella
aspergillum_.)]
The skeleton is formed by small crystals and long threads of vitreous
silica, cemented together, during life, by protoplasm. They are
arranged in longitudinal and annular bands so as to form a long curved
cylinder, about nine to twelve inches long, the walls of which are
about one inch in thickness. The threads and bands are interwoven with
the greatest regularity, and when the skeleton is freed from the
adhering organic matter, it looks extremely beautiful.
The mode in which the intersecting bunches of crystals are connected
is shown in Fig. 2. The upper end of the cylinder is closed by a
perforated cover, which probably has given rise to the name of the
sponge. The upper portion of the cylinder is surrounded by a few
irregular, annular masses of organic tissue, which adheres loosely
only to the skeleton. The lower end is formed by a bunch of long
threads, rooting firmly in the ground.
[Illustration: Fig. 2.--SPONGE CRYSTALS MAGNIFIED.]
Up to about ten years ago the price of specimens of this sponge was
very high. At that time, however, a colony of Eucleptellas was found
near the cities of Cebu and Manila, in the East Indies, in a depth not
exceeding 100 fathoms, and since they have appeared in larger
quantities in the market. It is remarkable that, contrary to their
habits, these organisms have immigrated into regions to which they
were totally unaccustomed. Yet it must be regarded as a greater
curiosity that they have been accompanied to their new abode by a few
animals living in equally deep water and never met with before at
depths less than three or four hundred fathoms. Among these animals is
a _Phormosoma_ (water hedgehog), noted for its long spines.
Glass sponges are not confined to tropical regions. They are met with
in latitudes as high as the Färöe Islands, where the beautiful
_Holtenia Carpentaria_ abounds. It is represented in Fig. 3. Its
cup-shaped skeleton is similar in structure to that of the
_Eucleptella_; numerous crystalline needles protrude from the surface
of the upper part. Lately some specimens of _Holtenia_ have been found
on the coast of Florida.
[Illustration: Fig. 3.--HOLTENIA CARPENTERIA.]
Glass sponges serve as dwellings for numerous animals, especially
crustaceæ. A small shrimp inhabits the tubes of the _Eucleptella_, a
male and a female generally living together. They are shut up as in a
prison in their crystalline home, as they are generally too large to
pass through the meshes formed by the bundles of crystals. It was
formerly believed that these skeletons had actually been built by the
shrimps, and we can find no explanation for this curious circumstance,
other than that the shrimps entered these habitations while very small
and became too large to leave them.
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