Protostomia

The protostomiens ( Protostomia ) are one of two divisions of the Bilatériens.

The characterization is based on criteria embryologic. The name, which means in Greek stops in first , describes their principal characteristic:

at the time of the Embryogenesis, after the phase of Gastrulation, the Blastopore will become the Bouche, the anus requiring the boring of a pore secondarily to the installation of fabrics.

The cellular cleavage of spiral type at the time of the Segmentation (the first divisions of egg) and the early determination of the cells belong to the other fundamental characteristics. Indeed, if one removes a cell at the stage 4 cells of the embryo, this one dies unable to develop contrary to the Deutérostomien S at which the cells can be reprogrammed until rather late during the development.

Finally the coelome is schizocoele. The Mésoderme form right after the Gastrulation a small mass which surrounds the Blastopore, between the Endoderme and the Ectoderme. This originally compact mass will grow hollow to form the Cœlome.

The nervous system is ventral except for the cephalic ganglion. The animal is of hyponeurien type with a collar périœsophagien. The larva Trochophore is characteristic.

The skeleton when there exists (Arthropode S) is a Exosquelette.

Note: the spiral embryonic segmentation is not general with all the protostomiens but it there forever of radial segmentation. The protostomiens are a very diversified group which contains the majority of the bilateria. This very vast group is him even divided into two groups as well by the study of the anatomy as by molecular biology. However, the two methods give very different trees which are not recut.

Internal bonds

Deutérostomien

External bonds

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