THE HUMBLEST OF ANIMALS--SPONGES
At the foot of the arrangement of phyla in the metazoa stand the
Porifera, or sponges, fixed, plantlike, queerly shaped beings living
in the sea, except one family in fresh waters, and abundant in all
the warmer parts of the world on rocky bottoms. Whatever its size or
shape, a live sponge (of which the commercial article is the more or
less perfect skeleton) is coated with a thin fleshy membrane perforated
by minute "inhalant pores" and larger holes termed "oscula," or
mouths. Through the inhalant pores the sea water, with its burden of
microscopic food, enters one of many spaces beneath the surface from
which incurrent canals penetrate the interior of the sponge, constantly
branching and growing smaller until lost to sight. The fine tips
communicate with small cavities lined with cells that are fitted to
seize and assimilate the nourishment brought them by the water. From
these rudimentary stomachs go similar excurrent ducts that unite near
the surface into trunk canals that carry out the used water and waste
products. This system of circulation, bringing nutrient water strained
through the pores, and expelling it forcibly after it has been cleared
of food value, is kept in motion, with occasional periods of rest,
by the action of "flagellate cells" that line certain tracts in the
canals. These are elongated cells from which project whiplashlike
filaments, one to each cell, whose movements in concert "resemble
those which a very supple fishing rod is made to undergo in the act
of casting a long line"--the movement being much swifter from without
inward.
Beneath the outer skin, and all among the canals and cavities, is a
filling of gelatinous materials, largely protoplasm, in which are
formed great numbers of variously branched and strengthening spicules,
of limy material in one group, and in others of a flinty or glassy
nature, or in the absence of these, a network of "spongin," such
as forms the skeleton of our common washing sponges. Spongin is a
substance allied to silk in chemical composition, and the threads are
felted together in such a way as to form a firm, yet elastic structure.
"In some Noncalcarea, which are devoid of spicules, the place of these
is taken by foreign bodies--shells of Radiolaria, grains of sand, or
spicules from other sponges. In others again, such as the Venus's
flower basket (Euplectella), the glass-rope sponge (Hyalonema), and
others, the skeleton consists throughout of siliceous spicules bound
together by a siliceous cement."
Sponges are reproduced both by budding in some form, which is an
asexual way, and by the sexual method of eggs and male cells; these are
formed in the same sponge, but rarely at the same time, and the early
stages of development are passed in a brood-cell within the body of the
parent sponge. Finally, the embryo escapes through one of the outgoing
canals, swims about awhile, becomes thimble-shaped, and settling down,
fastens itself by the closed end to some patch of mud, a rock, dead
shell or seaweed, closes the open end of the "thimble," and proceeds
to grow.
Sponges do not appear to be eaten by fishes or anything else. Countless
lower animals, such as marine worms, mollusks, and so forth, burrow
into them, however, in search of shelter; and in reversal, certain
small sponges, such as the cliona of our shores, burrow into the shells
of mollusks, which explains the honeycombed appearance of many of the
shells picked up on the beach. Sponges have a large part in that very
interesting and widespread phase of marine life called "commensalism,"
in which two animals become intimately associated in a mutually
beneficial way, and are thus spoken of as messmates. Some kinds of
sponge are never found growing except on the backs or legs of certain
crabs; the sponge conceals and protects the crab, while itself benefits
by being carried from place to place, with constantly new changes of
fresh water and food. This sort of partnership occurs in many different
groups of marine animals.
The capture and preparation of sponges for market employ thousands
of men and boats in the eastern Mediterranean, whence the best are
derived, and in the West Indies and Gulf of Mexico, where the sponges
are of a coarser kind, and are gathered and prepared by rougher
methods. They are taken commercially also in other seas, and frequently
dredged from vast depths.