COMPARISON OF BRYOZOANS AND CORALS
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Marine organisms called "corals" and freshwater and marine
"bryozoans" belong to separate animal phyla. The phylum is the
highest (greatest difference) category for classifying organisms,
and corals and bryozoans, although generally alike in many
aspects of their construction, are very different kinds of
creatures. Corals are in the phylum Cnidaria while bryozoans are
in the Ectoprocta (a smaller group, the Entoprocta, are also
called bryozoans but do not build large colonies). Like the
colonies of each group, the principal biological "units" that
build a coral or bryozoan colony are superficially similar in
appearance. The coral unit is the polyp while the bryozoan unit
is the zooid. Both have tentacles used to capture prey and their
bodies are permanently fixed to the colony mass, i.e. sessile.
Both coral polyps and bryozoan zooids are able to extend from and
contract within the colony matrix. However, closer examination
reveals dramatic differences in the two organisms. The coral
polyp is a very simple beast, its body being solid and composed
of only tissues. A cavity in the center of the animal is used
for digestion but is in no way a true stomach or intestine. The
bryozoan zooid, on the other hand, is a complex animal with
tissues, organs and a distinct body cavity that has nothing to do
with digestion. Food is processed through a distinct stomach and
an intestine, constituting a complete organ system, and passes
out of the body through an anus. In their own way, both
creatures generate large colonies of themselves. The most
familiar to all of us of course are corals in tropical oceans
which construct massive reefs. While not as dramatic, freshwater
bryozoan colonies live a similar lifestyle, forming large,
extended colonies that can cover large portions of the substrate
in freshwater habitats.
