Articulated animals
The division of living beings into systematics is a continuous subject of research. Different systematic classifications exist side by side and one after the other. The taxon treated here has become obsolete due to new research or is not part of the group systematics presented in the German-language Wikipedia.
In the concept of arthropods (Articulata) are the two taxa annelids (Annelida) and arthropods (Arthropoda) together. The group was first described by Georges Cuvier in 1795 at a lecture to the Société d´Histoire naturelle de Paris as one of the main groups of the animal kingdom (in Cuvier's concept as one of a total of four such main groups).
In the period that followed, the Articulata were considered to be one of the morphologically best secured communities of descent within the Protostomia . It was not until a molecular biological investigation from 1997 that the suspicion arose that annelids and arthropods were not sister groups, ie the assumption of a taxon "Articulata" could be invalid. This result has been repeatedly confirmed at the molecular biological level, so that the arthropods are now regarded as part of the new taxon molting animals (Ecdysozoa).
Moulting animals (Ecdysozoa) instead of articulated animals (Articulata)
On the basis of molecular biological data, the arthropods can be grouped together with the Cycloneurali to form molting animals. Cycloneuralia are Nematoda ( roundworms ), Nematomorpha ( string worms ) , Loricifera ( corset animals ), Priapulida (priapulidae ) and Kinorhyncha ( hookweed ). The gastrotricha ( belly curls ) that used to be included are now considered to be flatworms .
The taxon molting animals can only be supported to a very limited extent morphologically. Similarities ( synapomorphies ) of the cycloneuralia with arthropods are:
- The possession of a three-layer alpha cuticle with the participation of chitin , which is enlarged in the formation phase after a molt.
- Moulting is induced by ecdysone (a steroid hormone ).
- The loss of locomotor cilia
The most important similarities between arthropods and annelids are:
- A rope ladder nervous system with a cerebral ganglion in the head and a pair of ganglia with an anterior commissure in each segment.
- The division of the body into a front end (acron), a rear end (pygidium) and a number of originally homonomous segments.
- A similar formation and anatomy of these segments, which originally all have a pair of coelom sacs , a few metanephridia as excretory organs, a pair of ganglia of the rope ladder nervous system and a pair of extremities (or their precursors, parapodia).
- A cuticle with bristles.
Although in the higher (or Eu-) Arthropoda the homogeneity of the segments was greatly modified, ontogenetic studies still support the assumption of homology. The segments of the arthropods, like those of the annelids, are embryonic. In the Cycloneuralia there is a group that has an articulated physique, the Kinorhyncha . Due to the lack of evidence of homology, however, their body sections are not referred to as segments, but as zonites, as they have neither excretory organs nor limb precursors. In addition, their nervous system consists of 8–12 medullary cords, not a rope ladder nervous system as in articulates. However, since 18S rDNA studies also group the nematodes and relatives into the arthropods, neither of the two concepts can be regarded as definitive.
There is also a group of Echinodermata , as well as a taxon of castle-bearing brachiopods (arm pods) called Articulata.
source
Special zoology part 1: Protozoa and invertebrates, W. Westheide and R. Rieger (Eds.) Spectrum 2006.
Remarks
- ↑ Aguinaldo et al. 1997
literature
- AM Aguinaldo, JM Turbeville, LS Linford, MC Rivera, JR Garey, RA Raff, JA Lake: Evidence for a clade of nematodes, arthropods and other moulting animals. Nature 387, 1997, pp. 489-493.
- B. Klausnitzer, K. Richter: Tribal history of the articulated animals. Die Neue Brehm-Bücherei 541, Wittenberg Lutherstadt 1981, A. Ziemsen Verlag.
