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The
Diversity of The World of Life
Animals
(Metazoa)
Moss Animals etc.
(Bryozoa)
Representatives
Moss
Animals
Biology
APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE
4,000
ENVIRONMENTS
Although a few are freshwater species, most bryozoans are marine, attached to rocks and
other surfaces.
DESCRIPTION
The typically microscopic body is box-like, given shape by
an external "exoskeleton", typically composed of "chitin"
(a horny substance secreted by the animal, like the chitin in the
exoskeleton of an arthropod)
plus "calcium
carbonate" (a stony mineral concentrated from seawater). There
is an opening in the box-like body, sometimes covered with a hinged "operculum"
(lid), out of which protrudes and retracts a "lophophore", a
feeding organ composed of a ring of ciliated tentacles growing out from around the mouth
of the moss animal, living within the "box".
Each tentacle contains finger-like extensions of the "coelom"
(the membrane-lined cavity outside of the gut that fills most of the body).
Bryozoans live together in colonies, the individual
"zooids" cemented to each other and to the substrate (rocks,
etc.) by the backs and sometimes the sides of their
"boxes" -- the opening is actually located on the "ventral
surface" (the "underbelly", so to speak) of the moss animal
("lying on its back").
The colonies thus grow into crust-like or plant-like
("mossy") forms. There
is typically a "division of labor" between specialized zooids
within a
colony -- most are feeding, some may be supportive (such as those composing
stem-like "stolons"), some may be defensive, and some may be
entirely reproductive.
FEEDING HABITS
Moss animals are filter-feeders, catching food particles by
means of the ciliated tentacles of the lophophore.
MOTION
Although the "boxes" are cemented to the
substrate, well-developed, strategically located muscles withdraw and
protrude the lophophore (and close the operculum, if present).
To withdraw the lophophore, "retractor" muscles simply
pull the gut and the lophophore back into the box. To extend the
lophophore, "transverse" muscles pull-in either on the side walls
of the body or on a flexible membrane on the face of the box -- either
method increases the water-pressure within the coelom and "squirts"
the lophophore out through the opening.
DIGESTION
The U-shaped gut is "complete", with
both a mouth and an anus, both opening through the hole in the
exokeleton.
A water current, carrying food particles, is drawn-in through the funnel-like
lophophore by the action of the tentacular cilia, which then catch and
transport the food
particles in through the mouth and into the pharynx.
Between the pharynx and the anus, the "U-shaped"
gut is typically differentiated into a simple esophagus, stomach, and
intestine.
RESPIRATION
There are no gills in these tiny animals -- dissolved
gases and other materials simply diffuse by osmosis, across the selective cell membranes.
CIRCULATION
There is no circulatory system in these tiny animals -- dissolved gases and other materials simply diffuse by osmosis throughout
the interconnecting coelomic fluid in an entire bryozoan colony!
EXCRETION
There are no kidneys in these tiny animals -- nitrogen-rich and other wastes simply diffuse by osmosis out through the
again selective cell membranes.
COORDINATION
The
growth, development, and activities of bryozoans are under genetic and
hormonal control, influenced by the environment.
A
"ganglion" (nerve knot) and nerve ring, around the mouth,
coordinate the actions of the tentacles and the rest of the body.
REPRODUCTION
Moss animals can reproduce asexually, by the
"budding" of new individuals.
In a typical
colony, the outermost individuals are newly budded
and developing; the next individuals inward are fully developed and
functioning; the next individuals inward are apparently dying; and the
innermost individuals in the colony are actually regenerating, a new gut
and lophophore forming from the surviving body wall.
Moss animals can also reproduce sexually.
Each individual is "hermaphroditic" (having the gonads of
both sexes) -- an advantage for animals that cannot travel about in
search of a mate. The gametes
are discharged from the gonads into the coelom and out through a pore in
the lophophore tentacles or through a pore near the lophophore, as on a
stalked "intertentacular organ" -- it is in this organ or in
the water current of the lophophore that the eggs are typically fertilized.
The fertilized eggs and developing embryos are typically brooded in
special brooding chambers within reproductive members of the colony.
The larvae are planktonic (microscopic and drifting) and
somewhat like the "trochophore"
larvae of mollusks and annelids, with
whom bryozoans
probably share a common ancestor. After traveling to new locations,
the larvae settle down and start new
colonies of the "sessile" (stationary) adults, via asexual
budding (as described above).
Animals
(Metazoa)
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