A General Introduction.
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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.