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The Heliozoa. 
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Protozoa: Home Amoebae Ciliates Flagellates Heliozoa

  The Heliozoa.

The heliozoa are radially symmetrical unicells ( see diagram of Actinosphaerium) characterised by the possession of long, slender cytoplasmic arms called axopodia, stiffened by a complex structure of microtubules. Cytoplasm flows continuously back and forth along the axopodia, conveying bacteria and anything else which adheres to them back to the cell body. Larger prey such as algal cells may become enmeshed in the axopodia and similarly incorporated into the cell body.

"Heliozoa" is another example of a grouping based more upon appearance than a close evolutionary relationship between the organisms contained in it. These freshwater creatures, called "sun animalcules" by early microscopists, are now classified in the Actinopoda along with the exclusively marine Radiolaria.

They live among submerged decaying plants in freshwater habitats, and when found can be present in large numbers.

Actinosphaerium. Actinosphaerium. A very attractive microscopic subject, especially in darkfield. A brightly lit vacuolated cell body is surrounded by a radiating sunburst of exceedingly fine axopodia -- the very qualities which make it very difficult to photograph. This picture gives a good representation of the cell body, but only the thicker parts of the axopodia have registered on the film.
Darkfield: x100.
Heliozoan ingesting alga. This heliozoan is Actinophrys -- similar in appearance to Actinosphaerium, but about a tenth of the size. It has enmeshed and engulfed an algal cell of the same size as its own cell body.
Click for a diagram of Actinophrys.
Darkfield: x400.

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