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2 The Phylum "Bryozoa"
Morphology
Almost all bryozoans are colonial, composed of
a few to millions of individuals. The individuals reproduce themselves
normally by building new individuals asexually and adding it to
the colony. These new additions contain functioning individuals,
capable of feeding independently, yet remaining attached to the
rest of the the individuals in the colony. They also reproduce
sexually to form new colonies.
The individuals are called zooids. In living
state, a zooid is enclosed in a sheet of living tissue (zooecium),
that in most species secretes the calcium carbonate, building
the skeleton. The Bryozoan inside has a ring of tentacles (lophophore)
to capture small food particles from the seawater. When the tentacles
are retracted, the opening can be closed by the operculum.
Zooids are usually connected to each other by thin strands of
tissue.
In the colony there are several kinds of zooids
each with its own specialized function. The feeding zooids
are known as autozooids and the non feeding, specialized
zooids are known as heterozooids,which include egg and
sperm producing forms. Avicularia are small heterozooids
in which the zooecium and operculum form a beak like, snapping
structure that deters small predators. The are also specialized
cleaning zooids (vibracula) and zooids to strengthen the
colony and fill in space (kenozooids). A Bryozoan colony
is an integrated system which can behave like a single organism.
Bryozoan colonies can be either encrusting (on
substrates like algae, shells or rocks) or erect forms growing
upwards in the water column on its own. The latter form can be
massive (solid), foliaceous (sheetlike, with zooids
on both sides), dendroid (branch-like or tree-like), or
fenestrate (many branches joining and rejoining to form
a net-like shape).
Systematic
In the Class Gymnolaemata the zooids
are generally box- to sac-shapes or short cylinders, with the
long axis roughly parallel to colony growth direction. The zooidal
size is fixed early in ontogeny. The zooidal body walls are entirely
organic to rigidly calcified. There is interzooidal communication
by a funicular network through tissue plugged pores in vertical
walls. This class appears in the Upper Ordovician until Recent.
- Order Cyclomatida: Encrusting or erect
colonies; long autozooids with basal diaphragms and other
structures; laminated skeletal structure with many communication
pores; gonozooids are common and kenozooids are
present in some; extrazooidal skeleton may be present. (Upper
Ordovician - Recent)
- Order(?) Hederellida: A group of bryozoans
similar to cyclostomatids but of uncertain systematic placement.
(Late Ordovician - Carboniferous)
- Order Cryptostomatida: Erect bilaminar
sheets or tree-like colonies; short autozooids some with
basal diaphragms; no communication pores in the laminated skeleton;
kenozooids or extrazooidal skeleton may be present. (Lower
Ordovician - Upper Permian or Cretaceous?)
- Order Cystoporida: Colonies are encrusting
or erect; autozooids short and without basal diaphragms
or long and with basal diaphragms; most autozooids with
thickened strip (lunarium) of different microstructure along
one side in the thick-walled outer zone; laminated granular
skeletal structure, some with communication pores; kenozooids
are present in some and gonozooids are uncommon;
vesicular extrazooidal skeleton common or abundant. (Lower Ordovician
- Upper Permian or Cretaceous?)
- Order trepostomatida: Colonies are encrusting
or erect; autozooids generally containing basal diaphragms
and commonly other lateral structures; kenozooids are
common and extrazooidal skeleton is present in some; typically
laminated skeletons without communication pores. (Lower Ordovician
to Upper Triassic or Recent?)
- Order Fenestrida: Colonies are erect
and composed of narrow unilaminate branches; short autozooids
commonly with hemisepta but basal diaphragms only in very few;
no communication pores in primary zooids; typically granular
skeletal structure; various heterozooids are present
in some; extensive extrazooidal skeleton consisting of laminae
penetrated by small granular rods. (Lower Ordovician to Upper
Permian or Triassic)
In the Class Stenolaemata the zooids are
elongate cylindrical and continue to lengthen through ontogeny,
with the long axis at an angle to local colony growth direction.
The basal and vertical walls are rigidly calcified. In most Stenolaemata,
interzooidal communication occurs through space at the outer end
of vertical walls; through pores in vertical walls in some others.
This class appears in the Lower Ordovician until Recent.
- Order Ctenostomatida: Membranous or
gelatinous zooidal walls; orifice terminal and closed by pleated
collar; heterozooids are absent or only kenozooids
are present. (Upper Ordovician - Recent)
- Order Cheilostomatida: Zooidal walls
calcified, flexible or rigid; orifice frontal and
closed by proximal hinged operculum; heterozooids
are usually present; the suborders are based upon frontal calcification
and mechanism of lophophore protrusion. (Upper Jurassic
- Recent.)
The Class Phylactolaemata consists of colonies
without a mineralized skeleton. Phylactolaemata reproduce mainly
by asexually produced statoblasts. They live in freshwater and
are found in sediments from early Cretaceous until Quaternary.
© 1999
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