Multicellular animals

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Multicellular animals
Above: sponges, sea anemones, earthworms Below: porcelain crabs, yellow-breasted macaws, anubis baboons

Above: sponges , sea ​​anemones , earthworms
Below: porcelain crabs , yellow-breasted macaws , anubis baboons

without rank: Obazoa
without rank: Opisthokonta
without rank: Holozoa
without rank: Filozoa
without rank: Choanozoa
Empire : Multicellular animals
Scientific name
Haeckel , 1874

The multicellular animals (scientifically Metazoa , from ancient Greek μετα meta , German 'after' and ζῷον zóon 'animal') are a zoological taxon in which all multicellular animal groups are summarized. Today 1.2 million species of multicellular animals are known. With those not yet described, it is estimated that there are 10 to 20 million species. Of the species known to date, the arthropods make up over 80% with about a million, and within the arthropods the beetles and butterflies together make up half of the species. Arthropods and mollusks , the second most species-rich tribe with 100,000 species, comprise 90% of the species living today. The vertebrates represent 5% of the biodiversity of the animal world.

In scientific publications today the Metazoa are often used synonymously with the animals (Animalia), for the Metazoa and the unicellular representatives from their parent group , the new taxon Holozoa was formed in 2002 .


Metazoa is characterized by the fact that they are made up of several cells and the differentiation and specialization of these cells. In sponges , for example, there are different cell types that are used, for example, for food acquisition, breathing (collar whip cells) or for skeletal structure. Further features are a specialized gametes genesis, flagellated sperm , the cleavage ( cell division at the fertilized egg cell by pinch) and the genome of the mitochondria .

Differentiated tissue

The most important characteristic of the Metazoa is the multicellular body, which is not a colony of the same cells that could exist more or less independently of each other. The cells are specialized and morphologically and functionally differentiated. Similar cells communicate with one another by means of chemical signals, influence one another, work together and form tissue. The communication serves, among other things, to maintain the body shape and the shape of the organs.

Basic tissue types are the epithelium (lining tissue ) and the connective tissue . The epithelial tissue consists of a closed cell structure with almost no intercellular spaces. The cells are in function and structure often asymmetrical (polarized) and to each other in different ways connected .

Connective tissue, on the other hand, is more loosely structured, with isolated cells in the extracellular matrix . The extracellular matrix is composed of net-like arranged collagen fibers, water, and glycoproteins . The epithelium and connective tissue are separated from each other by the basement membrane .


The cilia of the sperm and other metazoic cells have striated roots and vertical centrioles , one of which forms the basal body of the cilia. Otherwise they have the same structure as all eukaryotic cilia.


Multicellular animals produce egg cells and sperm for sexual reproduction. Their gametogenesis is similar. A diploid mother cell gives rise to four haploid cells, either four sperm or the fertilizable egg cell and three pole cells, which normally disappear quickly. It is believed that the production of a large, nutrient-rich egg cell is too costly for each of the four cells to produce an egg cell.


After fertilization, the resulting diploid zygote initially divides into many small daughter cells ( blastomeres ) without cell growth taking place . In this furrowing the basic axes of the future emerge cell division called embryo and a first differentiation. The blastula arises from the furrowing , and invades in the gastrulation . In the course of further differentiation, the cells divide partly asymmetrically, a stem cell and a cell that differentiates further develop from one stem cell.

For a long time, the types of furrowing were used as a feature to relate the various animal phyla to one another. However, many tribes have developed their own type of groove, and the systematic importance of the different types of groove is difficult to assess.


The mitochondria of the Metazoa have a comparatively small genome with around 16,000 nucleotides on 37 genes . In comparison, the choanoflagellate genome is four times as long. In addition, the transfer RNA and the ribosomal RNA have an unusual structure.


The sister taxon of the multicellular animals - i.e. the actual animals in the modern sense of the word (Metazoa) - consists of the collar flagellates ( Choanoflagellata ). Animals and flagellates together form the group of Choanozoa . They are combined with the unicellulars of the Filasterea to form the community of descent of the Filozoa , which together with the unicellulars of the Pluriformea and Ichthyosporea form the group of Holozoa . The sister taxon of the Holozoa consists of the Nucletmycea , to which, for example, the mushrooms ( Fungi ) are assigned. They form the two branches of the Schubgeissler ( Opisthokonta ).

External classification of the Metazoa
  • Opisthokonta
    • Nucletmycea
    • Holozoa
      • Ichthyosporea
      • (unnamed)
        • Pluriformea
        • Filozoa
          • Filasterea
          • Choanozoa
            • Choanoflagellata
            • Metazoa

The most basal multicellular animals consist of the monophylum of the sponges ( Porifera ). Its sister taxon is called Epitheliozoa , in which all other animals are classified. The taxon contains the disc animals ( Placozoa ) and the tissue animals ( Eumetazoa ). The latter are divided into the groups of hollow animals ( Coelenterata ) and bilateral animals ( Bilateria ). The hollow animals include rib jellyfish ( Ctenophora ) and cnidarians ( Cnidaria ). The bilateral animals are divided into Xenacoelomorpha and kidney animals ( Nephrozoa ). The latter consist of primeval mouth animals ( Protostomia ) and new mouth animals ( Deuterostomia ).

Inner systematics of the Metazoa






























The inner systematics of the animals just presented contains the coelenterata concept. A clade of coelenterates from cortisacenas and cnidarians is shown as sister taxons of the bilateral animals. The figure could suggest a taxonomic security, which does not exist at the current state of research. In fact, the exact family relationships between the monophyletic groups of sponges, disc animals, rib jellyfish, cnidarians and bilateral animals are discussed. The Acrosomata concept postulates a group of comb jellyfish and bilateral animals called Acrosomata . The Planulozoa concept sees a community of descent in cnidarians and bilateral animals called Planulozoa . The Planulozoa concept is framed by the Parahoxozoa concept, in which the disc animals together with the Planulozoa form a clade called Parahoxozoa .

Concepts for the systematics of the Metazoa
Coelenterata concept Acrosomata concept ParaHoxozoa / Planulozoa concept
  • Metazoa
    • Porifera
    • Epitheliozoa
      • Placozoa
      • Eumetazoa
        • Coelenterata
          • Ctenophora
          • Cnidaria
        • Bilateria
  • Metazoa
    • Porifera
    • Epitheliozoa
      • Placozoa
      • Eumetazoa
        • Cnidaria
        • Acrosomata
          • Ctenophora
          • Bilateria
  • Metazoa
    • Ctenophora
    • unnamed
      • Porifera
      • ParaHoxozoa
        • Placozoa
        • Planulozoa
          • Cnidaria
          • Bilateria

Tribal history

The temporal origin of the Metazoa is still uncertain and controversial. This is because all fossils from the Precambrian , but especially all those for which an age before the Ediacarian has been given, are difficult to interpret and their classification is controversial. Numerous details of older fossils were later questioned by other researchers or even subsequently unequivocally disproved as remains of Metazoa. Age estimates based on the methodology of the molecular clock are even more unreliable, since for calibration they have to be based on reliably dated finds or events that have not been verified. Numerous dating with this method therefore resulted in age estimates that were far too high. Dating based on certain molecules in very old sediments, which have been used as biomarkers for certain groups, especially for sponges, is very controversial, since these compounds could have originated differently (or possibly only penetrated the strata to be dated as an impurity ). Interpretations based on the branching pattern of the basal multicellular animal clades , the last of the possible methods of dating, are problematic because there is no scientific consensus as to which of the groups is actually the oldest (i.e., the one that branched off first from the common parent group).

It is unclear what features a possible fossil trunk group representative of the Metazoa could have exhibited. Ax indicates the following characteristics as autapomorphies of the Metazoa : Separate sex ("gonochoric") organism made up of diploid somatic cells, haploid gametes and diploid zygote , oogenesis with an egg cell and three rudimentary polar bodies , sperm differentiated into a head with the nucleus and a flagellum a middle section ("neck" with centrosome with two centrioles arranged at right angles and four mitochondria ), during spermatogenesis four identical sperm cells emerged from each spermatocyte . The tissue of the somatic cells is only selectively permeable to certain substances due to dense cell-cell connections ( tight junctions ), which enables an internal milieu. There is an extracellular matrix that contains collagen fibers . The development from the germ cell takes place through radial furrowing . For the fully grown animal, he assumes a flattened hollow sphere ( blastula , placula) as the organizational form, which consists of flagellated cells on the outside for locomotion and food intake and uncultivated cells on the inside.

Of the features mentioned, with some restrictions only one, radial furrowing, can actually be observed in fossils. Fossils of structures that are interpreted as cells with radial furrows (possible embryos of multicellular cells) are from the Doushantuo Formation from southern China, the so-called Weng'an Biota , whose age is dated to around 570 to 600 million years. Fossils of other possible multicellular animals are also available from the same formation, but their interpretation is controversial. This is probably the oldest convincing fossil record of Metazoa. Even older fossils (or trace fossils ) have been proposed in large numbers, but most of them could be identified as fossils of multicellular algae or cell threads of prokaryotes (such as cyanobacteria ), the rest are extremely dubious and controversial.

In the younger layers of the Ediacarian , after the Gaskier Ice Age about 580 million years ago, the fossils of the enigmatic Ediacaran fauna can be found . The interpretation of these is completely controversial, but some of the famous fossils such as Kimberella , Dickinsonia or Eoandromeda are accepted by most researchers as Metazoa. The oldest representatives of the animal phyla living today, the assignment of which is beyond doubt, actually only date from the Cambrian , about 541 million years ago. In the Lower Cambrian Chinese Chengjiang faunal community , around 520 million years ago, almost all animal phyla that have the potential for fossilization are already represented. This comparatively sudden appearance of the "modern" fauna is known as the Cambrian explosion .

Fossils of multicellular animals are therefore with some certainty no earlier than in the Ediacarium , after the end of the Marino Ice Age . However, since the first representatives, as soft-skinned individuals of very small body size, could hardly have been fossilized (and, even if they were actually discovered, would probably have too few features for a reliable classification), an actual one also appears after the estimates using the molecular clock method Formation already in the Tonium , before the Ice Age of the Cryogenium , by no means unlikely.


Web links

Commons : Animalia  - collection of pictures, videos and audio files

Individual evidence

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