In conclusion of my thoughts, I will try to understand what extraembryonic mesoderm is and how it differs from the embryonic mesoderm itself.
The larva has three embryonic membranes. The left and right gonads are separated by mesoderm, which does not dissolve during the development of the embryo throughout the entire time, while the sharka ruptures and fills the internal space to form a sharka. In addition, the sharka passes through the larva’s body into the stomach and exits through special tubes - hypostadia. The splitting of the mesenchymal layer of the mesoderm gave rise to the organs of the abdominal cavity - coelomia and neocoelomia (nephroepiboly and ecdysostotic system). So