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A General Introduction.

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Part of a colony of the bryozoan Lophopus.

  The Bryozoa.

The bryozoa are sessile colonial aquatic animals. The individuals are called zooids, and they reproduce by a process of budding. Of the 5000 or so known species, only about 50 are found in fresh water.
Although the marine bryozoa are most common in the shallow waters of the continental shelf, they have been found at ocean depths of 8000m. The colonies they form can be large gelatinous or calcareous patches involving many thousands of individuals, or long branching chains resembling plants (the "polyzoa" and "moss animalcules" of the early microscopists). This diagram (54 KB) is of Pectinatella, a typical freshwater genus.

Though the individual zooids superficially resemble hydroid polyps such as those of the freshwater Hydra, they are considerably more complex. The freshwater bryozoan Lophopus, shown in these pictures, shows the structural features common to all brozoans -- most conspicuously the numerous long, ciliated tentacles and their supporting structures, collectively called the lophophore, which generates the feeding current and oxygenates the animal.

They provide one of the microscope's most compelling spectacles, especially in darkfield when the movements of suspended particles shows the feeding currents clearly.

Here is a diagram of the similar bryozoan Plumatella.


Lophopus. A small colony of Lophopus, with part of another visible to the right. It is attatched to a minor branch of a water plant. The individual extended zooids stand about 1mm high. The green plants in the background are the leaves and rootlets of the common duckweed, Lemna.

The oval, seed-like objects seen in the mass of the colony are called statoblasts. They are produced asexually by budding and are capable of surviving winter. In spring the two halves of the armoured outer layers separate, and the emerging zooid produces a new colony by budding. A sexual phase, usually in summer, also occurs.
Nikon F with 55mm macro lens on bellows.
Lophopus. Two zooids from the Lophopus colony above. The U shaped form of the lophophore and its ciliated tentacles is seen. The feeding current generated by them brings food particles -- mostly algal unicells and protozoans -- to a mouth placed at the bend in the U. When disturbed, the lophophore and its tentacles can be quickly retracted into the gelatinous base of the colony.
Rheinberg: x20.
Lophopus. Two small colonies of Lopophus. The gelatinous base of the colony is not firmly attatched, but is capable of drifting along the branch over a period of time.
Nikon F with 55mm macro lens on bellows.

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