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The
Diversity of The World of Life
Animals
(Metazoa)
Sponges
(Porifera)
Representatives
Typical
Sponges
Glass Sponges
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
8,000
ENVIRONMENTS
Most
sponges are marine species; some are freshwater. Sponges
are not only "benthic" (living on the bottom) but also "sessile"
(attached to the bottom).
DESCRIPTION
Although they are truly multicellular organisms, not simply
colonies of independently functioning and reproducing cells, sponges have no
true "tissues" (specialized, coordinated assemblages of cells),
let alone "organs" (specialized, coordinated assemblages of
tissues) -- sponges are structurally and functionally amongst the very simplest
animals.
Between porous, skin-like layers of very flat cells ("pinacocytes"), there is a
gelatin-like "mesenchyme", containing "amebocytes"
("amoeboid", that is, amoeba-like,
cells), which produce stony "spicules" (from salts in the sea) and/or fibrous
proteins (the stuff of natural bathroom sponges) that give shape to the "hollow" body
(hollow like a flat to round or lobed
jar). In "glass sponges", the spicules are glassy and fused into a lacy skeleton.
Sponges
are typically "macroscopic" (as opposed to microscopic), occasionally a meter or more
across, the larger forms typically more elaborately infolded.
Sponges are often very colorful, as from pigments produced by the
amebocytes or symbiotic algae.
FEEDING HABITS
Sponges are "filter-feeders"; they trap plankton and other particles of food drifting in the "feeding
current" of water drawn in through the body of the sponge (See below).
MOTION
Although the adult
sponge animal does not move about from where it is anchored to the seafloor, the
propeller-like "flagella" of
"collar cells" (very reminiscent of collar
flagellates), embedded within the inner wall of the body of the
sponge,
draw-in a steady stream of water through tiny pores in "porocyte"
cells within the body wall. After
passing through the central body cavity (the "spongocoel"),
sometimes with interconnected canals, the water current exits the body of
the sponge through one or more "oscula" (relatively
large, adjustable openings).
DIGESTION
Plankton and other
food particles drawn-in by the feeding current of water through the body of the sponge (See
above) are trapped by a
collar-like fringe of fingerlike "microvilli" on each collar
cell.
In some species, there are elaborate infoldings of the body wall, to increase the intake of dissolved food
particles, and the output of dissolved waste products. After
passing through the central body cavity or canals of a sponge, the water current, carrying wastes, exits through the
one or more oscula.
There
is no true digestive "system" (specialized, coordinated assemblage of
organs) within the body of a sponge, as there is in higher animals. Food
materials typically diffuse by "osmosis" (through selective cell
membranes) from cell to cell within the body of the sponge; however, foods not digested by
collar cells are absorbed by the amoeboid cells, which can
process, store, or transport the food throughout the body of the sponge.
RESPIRATION
"Respiration" typically refers to the biochemical
process by which the energy in food is released, typically with the
consumption of oxygen gas; however, in higher animals, respiration can
also refer to the physical processes by which air with oxygen is taken
into the body and air with carbon dioxide (a waste product of respiration)
is expelled.
Oxygen dissolved in the water of the feeding current (See
above) enters the
body of a sponge by osmosis, and
carbon-dioxide likewise dissolves in the water
and exits the body of the sponge. As
for foods, the elaborate infoldings of the body wall of some species help increase the exchange of
gases between the
sponge and its environment.
Because
there are no well-developed tissues, let alone organs, there is no true respiratory "system",
as in higher animals:
Dissolved gases simply diffuse from cell to cell within the body of a sponge.
CIRCULATION
The feeding
current of water (See above) does pass through pores in the body wall and out through the central body cavity or canals of the
body of a sponge; however, with no
well-developed tissues, let alone organs, there is no true circulatory "system"
within the body of a sponge, as in higher animals. Materials simply
diffuse by osmosis from cell to cell within the body; and materials
not digested by the collar cells are
absorbed by the amoeboid cells, which process, store, or transport
the food throughout the body of the sponge.
EXCRETION
Excess water and
dissolved salts are excreted from the body of a sponge simply by osmosis into
the feeding current of water passing through the body (See above) -- because
there are no well-developed tissues, let alone organs, there is no true
excretory "system", as in higher animals.
COORDINATION
The
structures and functions of sponges are controlled by genetics, as modified by environmental factors.
The activities, growth, and development of sponges are probably also
coordinated by hormones (in effect, chemical messengers), as
suggested by the observation that sponge cells separated from one another can automatically reassemble themselves into the sponge
body.
With
no well-developed tissues, let alone organs, there is no nervous "system",
as in higher animals.
REPRODUCTION
A sponge can
reproduce asexually, from its active amoeboid cells, from broken-off body parts,
from specialized "buds" it may
produce, or from "gemmules" (hardened cell bodies, as produced
by the relative few freshwater species, that can survive harsh periods,
as in inland waters).
Most
sponges can also reproduce sexually. Most sponges are "hermaphroditic"
(each individual is both male and
female) -- a definite advantage for these immobile animals, unable to seek-out a
mate.
"Gametes"
(sex cells) are produced in the mesenchyme from amoeboid cells, by
"meiosis" (cell division that cuts the number of chromosomes in
half): There are no specialized "gonads" (gamete-producing
organs), as in higher animals. Typically, great clouds of the sperm
cells (male gametes) are released into the water current passing through the body of the sponge and
out into the environment. After
being taken in with the water current of another sponge, sperms sexually fertilize the
eggs (female gametes) within the body of that second sponge. The fertilized eggs may
either be released immediately into the water stream
or be borne for a while within the body of the parent. The solid or hollow, multicellular "larva" (young) of a sponge uses its many
propeller-like flagella to swim to another location, in which it will settle-down.
There, the young sponge turns its body inward upon itself (in a process
called "gastrulation") and, thus, forms the central cavity of a
new sponge.
Animals
(Metazoa)
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