Articulata hypothesis
The concept of arthropods ( Articulata ) the two taxa segmented worms ( Annelida ) and arthropods ( Arthropoda ) are compiled. The group was first described by Georges Cuvier in 1795 in a lecture before the Société d' Histoire naturelle de Paris as one of the main groups of the animal kingdom ( in Cuvier's concept as one of four such major groups ).
In the period following the Articulata regarded as one of the morphologically most secure clades within the Protostomia. Only a molecular investigation of 1997 has raised the suspicion that annelids and arthropods are not sister groups, ie the adoption of a taxon " Articulata " may be invalid. This result has been repeatedly confirmed at the molecular level, so that the arthropods are a part of the new taxon Ecdysozoa today.
Ecdysozoa instead Articulata
Due to molecular biological data can best be summed up with the arthropods Cycloneuraliern to the Ecdysozoa ( molting animals ). As Cycloneuralia be summarized Nematoda (roundworms ), Nematomorpha ( hairworm ) Loricifera ( Corset animals ), Priapulida ( Priapswürmer ) and Kinorhyncha ( Hakenrüssler ). The previously calculated to Gastrotricha ( Bauchhärlinge ) is one of today's flatworm -like.
The taxon Ecdysozoa can, however, morphologically only support very limited. Similarities ( synapomorphies ) of the Cycloneuralia with Arthropoda are:
- Possession of a three-layer alpha - chitin cuticle that is magnified by skins.
- The molt is ecdysone induced ( a steroid hormone).
- The loss of locomotor cilia
The main synapomorphies of arthropods and annelids are:
- A rope ladder nervous system with a cerebral ganglion in the head and a pair of ganglia with ventral commissure in each segment.
- The division of the body into a front end ( Acron ), a rear end ( pygidium ) and a number originally homonomer (segments).
- The identical form and anatomy of these segments that originally all a pair Coelomsäcke, a few Metanephridien as excretory organs, a pair of ganglia of the nervous system rope ladder and a pair of limbs ( or their precursors, parapodia ) hold.
- A cuticle with bristles, the structure conserving chitin containing component.
Although the higher (or Eu ) arthropods homonomy of the segments was greatly modified to support ontogenetic investigations continue acceptance of homology. Embryo to be created, the segments of arthropods such as the annelids. The Cycloneuralia there is a group that has a structured physique that Kinorhyncha. Due to lack of homology evidence their body sections are, however, not referred to as segments, but as Zonite because they have neither excretory organs, nor extremities precursor. There is also her nervous system from 8-12 medullary cords, not from a rope ladder nervous system as Articulaten. However, as well as 18S rDNA studies can group the nematodes and arthropods related to neither of the two concepts are considered final.
There is also a group of the Echinodermata, and a taxon closed bearing brachiopods ( brachiopods ), called Articulata.
Source
Systematic Zoology Part 1: protozoa and invertebrates, W. West Heath and R. Rieger (eds. ) Spectrum 2006.