Bilateria

The bilatériens ( Bilateria ) are one of the two great morphological and phylogenetic subdivisions of the Eumétazoaire S (in opposition to the Radié S). They are characterized by a bilateral symmetry at the origin of their name, the presence of three fundamental cellular layers (they are Triploblastique S) Ectoderme, Mésoderme and Endoderme (instead of two for the others métazoaires Diploblastique S), a digestive tract with two openings (the mouth and the anus) and differentiated bodies. The Nervous system does not consist any more of a nervous plexus, but the Neuron S are gathered within various structures: ganglia, nervous chain or brain.

There exist two great divisions of bilatériens according to whether, during the embryonic development, the mouth appears before the anus or the opposite: the Protostomien S ( stops in first ) which are mainly the Arthropode S and the Mollusque S, and the Deutérostomien S ( stops as a second ) which are the Vertébré S and echinodermatous S.

Some bilatériens as the echinodermatous S have a bilateral symmetry only to the larval stage which they once lose adult for a radiate morphology. In the same way, the Gastéropode S lost bilateral symmetry their internal bodies.

The most former known representative of the bilatériens is Vernanimalcula guizhouena .

The vermiform stage

Functional characterization of the vermiform types

The organization of the " type; ver" is a new answer brought to the key question of the animal kingdom: how to eat? Basic strategy of the organizations of the " type; ver" (vermiform) is to move to go to seek food, instead of waiting until it passes to range. This strategy in particular makes it possible to exploit organic waste, which can be with high food value, but do not move.

Passed the course of sponges and the polyps, all the complex organizations are Bilatérien S, which derive from a fundamental diagram: the tube. The development is organized around an axis head/tail on the one hand, and back/belly on the other hand. These two axes lead to an overall plan where the right-sideds and left tend to being symmetrical, of or their name of bilatérien.

Clean characters of the bilatériens

Principal the " invention" bilatériens is invisible: they are the genes of development HOX, which control the development differentiated from the bodies from before behind, and their coordination with the nervous system. This system allows the appearance of a differentiated anatomy, and the operational coordination of its activity by the treatment of flow of information, which opens the way with a evolutionary Céphalisation.

The simplest worms mark an additional qualitative stage compared to the cnidaires: the nervous cells are organized in a coherent nervous system, prototype of what will become the brain in the higher animals. It is this capacity of displacement and reaction to the environment which makes consider that the vermiform degree of organization is the first stage indisputably " animal".

The second major invention of the worms (absent in the flat worms) is the presence of a food channel and a digestive function: at an end, a mouth absorbs food, with the other, an anus excretes waste. Between the " tube extérieur" who forms the skin (Ectoderme) and the " tube intérieur" (Endoderm) that is the food channel, an intermediate fabric, the Mésoderme, can develop and form increasingly complex internal bodies.

The invention of the digestive tract starting from the ancestral gastric cavity twice seems to be made. At the Protostomien, the two openings of the food channel are formed starting from the Blastopore, which the lips approach to form a channel by longitudinal welding. At the Deutérostomien S, the opening of the Blastopore becomes the Anus, the food channel being formed by a later boring which will evolve to the mouth.

" vers" are at the origin of one second major invention of the evolution: the segmentation (Metamerism). This invention also seems to be made in several different branches.

Radiative explosion of the worms

The formula " bilatérien" is a general-purpose type of organization, which was used as a basis for functional explorations and major animal groups:

the Mollusque S invent the raspy language, the Radula, which allows them brouter. They initially explored the function of protection which brought of the plates limestones on the skin. This formula of protection was not very conclusive, and is not any more the essential characteristic of molluscs.
the Arthropod S, segmented and having a chitin carapace, initially exploited the functionality which the walk and the gripping brought, made possible by articulated legs. This formula was an immediate success, leading to the radiative explosion of the arthropods.
the Poisson S acquired and developed the capacity with to swim in full water. Contrary to the two preceding groups, whose major functionality quickly caused an important radiative explosion, this group was characterized by an initially slow progression, and functionally not very conclusive. It is only while developing, with their turn, walk, with appearance of Quadruped S and conquest of dry land by Amniote S, that this group found the formula key which allowed him two successive radiative explosions, that of the Reptile S, and that of the Mammifère S.
the Echinoderme S lost the bilateral symmetry of origin in the adult form, and are generally with radiate symmetry.
Lastly, some bilatériens like the entoprocte S or the Tunicier S, gave up at the adulthood the essential functionality of the worms, mobility, to turn over to a fixed lifestyle characteristic of the sponge S.

Place des bilatériens in the animal types

External bonds

Simple: Bilateria

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