Heliozoa

The spherical heliozoan ( Heliozoa ) are single-celled organisms. They have special pseudopods ( Axopodien ) projecting radially in all directions. The Axopodien are not the locomotion, but the capture of prey as well as the increase of the water's resistance to slow a fall.

For a long time the Heliozoa were considered a natural kinship group, but in recent decades they have proven to be a polyphyletic, so unnatural group is thrown together from different kinship groups, so to speak:

The Rhizaria living in freshwater Desmothoracida, Helio Mona Dida and Clathrulinidae and marine Gymnosphaerida and now one of the Radiolaria ( radiolarians ) are counted Zanclea kind Sticholonche.

Do not dwell classify ( incertae sedis ) is the order Centrohelida, and from the group of Chromalveolata the Stramenopilen family Actinophryidae and some related genera to the heliozoan were counted.

Description

The cytoplasm consists of the outer, coarsely vacuolated ectoplasm and inner, finely vacuolated endoplasm. Most common are the envelope and skeletal formations that were important for the systematic classification of Heliozoa. Some forms are annealed foreign bodies such as diatom shells or grains of sand as shell structures recognizable ( Acanthocystidea ) under the light microscope on the outer cell envelope. Form or to build its case itself, the components formed thereby Other Heliozoa other hand, are very diverse but characteristic for each species. So you can find small balls, discs, plates, needles and hollow spines. The Clathrulinen for example, have similar radiolarians small latticed silica shells. Heliozoan are almost all freshwater residents. Preference is usually mesotrophic standing waters or slow-flowing ditches with dense vegetation.

Ingestion

Food intake is via the beam-like extensions of their cells, the Axopodien, the transport to the center of a grain flow. Both at the cell periphery as well as in Corticalplasma (inner cell membrane area) of the Axopodien there are numerous small moving grains, which are called Extrusomen. Extrusomen are cell organelles that may ejecting to certain external stimuli ( chemical and physical) out special ingredients. These substances cause sticking to the heliozoan that prey organisms ( such as paramecia ). Then, to wrap around the food, the protoplasm at the top of Axopodien may extend to include as quickly as possible in a vacuole around the booty ( phagocytosis ). Originally it was believed that direct contact with the radial Axopodien the prey would be paralyzed or stunned by a toxin. This conception has proved to be right, because even during the entire course of the enclosure of the prey in the food vacuole can be observed by light microscopy, a resistance of the trapped ciliates or flagellates. Every now and then they manage to even break away again.

The Axopodien the heliozoan seem in shape and size normally quite stable, but they can also be recovered as needed - rather, melted - be. This happens especially during and after ingestion. Some Heliozoa such as Actinophrys sol, forming so-called scavenger communities ( commensalism ) By fusing with other cells to jointly capture larger prey items. Here, two or more individuals may join together. In this way heliozoan large aggregates ( colonies) can form from many individuals. Similar to the amoeba the prey is trapped in a large food vacuole and subsequently digested together. After about 24 hours the individuals separate from each other and each "go" again his own way. Heliozoa mainly feed on other protozoa such as ciliates ( ciliates ), occasionally by small metazoans ( multicellular organisms ) such as water bears ( tardigrades ) and rotifers ( Rotifera ).