Spiralia

Spiralfurchung in the snail Trochus.

As Spiralia (Spiral forming taps ) are summarized a number of strains of the Bilateria ( animals with two sides ) that have a special kind of cleavage that Spiralfurchung. When Spiralfurchung four new cells in a spindle shape are formed obliquely to the axis of the egg.

The fertilized egg divides first two meridional (along the rotation axis of the ice), the division furrows run it from one pole to the other. There are four cells located in a plane and are designated by the letters A, B, C and D (Stage A of Taxobilds ) arise. The third division takes place in equatorial positions. There are two levels created, each with four cells, four large blastomeres, the macromers (1A - 1D) and four smaller cells, the micromeres (1a - 1d ) ( phase B of Taxobilds ). The micromeres are both in the furrows between two macromers and are compared with those rotated by 45 °. The four macromers divide again and form a second group of four micromeres (Phase C of the Taxobilds ). The micromeres divide, so that the egg is after four divisions of 16 cells ( phase D of the Taxobilds ).

Untertaxa

For the following taxa Spiralfurchung can be clearly observed, so their allegiance to the Spiralia is certain:

  • Segmented worms ( Annelida )
  • Molluscs ( Mollusca )
  • Spray worms ( Sipuncula )
  • Ribbon worms ( Nemertini )
  • Flatworms ( Platyhelminthes )

Other taxa have a Spiralfurchung in which is still not sure whether to the annelids and molluscs is homologous:

  • Gnathostomulids ( Gnathostomulida )
  • Calyx worms ( Kamptozoa )
  • " Rotifers ( Rotifera ) "

In arthropods ( Arthropoda ), there is no Spiralfurchung. They were considered to be relatives of the annelids (concept of arthropods ( Articulata ) ). The Spiralfurchung of ice in some crustaceans (water fleas ( " Cladocera " ), barnacles ( Cirripedia ) and illuminants crabs (Euphausiacea ) ) has arisen probably secondary.

In the horseshoe worms ( Phoronida ) a Spiralfurchung occurs in which the cells are offset by 45 °. Again, a secondary formation is assumed.

Organism Eumetazoa
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