wondered about the simplicity of the acoela body plan organization: is it the consequence of a loss, of regressions of characters during the evolution or the acoels preserved, for certain characters, an ancestral body plan organization? These flatworms were first assimilated to Turbellaria within the phylum plathelminthes, mainly on the basis of morphological resemblances. After numerous phylogenetics and phylogenomics studies, these acoela worms were placed within the subphylum Acoelomorpha phylum Xenacoelomorpha. The phylogenetic position of the acoela is unstable and has been the subject of much discussion: arguments place them at the base of the bilaterians, before the deuterostomians/protostomians dichotomy, while others place them rather at the base of deuterostomians. Regardless of their phylogenetic position within metazoans, acoela may have retained some ancestral traits of bilaterians during evolution and thus represent an object of study that would potentially help to better understand the diversification of body plan organization in animals with bilateral symmetry. The photosynthetic partner and the modus vivendi between the animal and the micro-algae Discovery and characteristics of the partner micro-algae Neither Geddes (1879), who observed the presence of starch and chlorophyll in the green cells present in the tissues, nor Delage (1886) and Haberlandt (1891) had formally been able to identify their origin and nature, however, suspecting micro-algae. In 1901, in Roscoff, William Gamble and Frederick Keeble began to study these green cells in hospite and tried to isolate them and put them into cultivation—with no success. In 1905, they observed non-symbiotic juveniles that were greening as they had hatched from cocoons, originally laid in unfiltered seawater, but transferred and incubated in filtered seawater. They hypothesized that the factor infecting juveniles and conferring the green color was probably on the surface or inside the cocoon. In a second experiment, they removed empty cocoons (post-hatching) initially contained in unfiltered seawater and then transferred and incubated them in filtered seawater. After 3 weeks, they observed in the filtered seawater a greening of these cocoons and the accumulation of unicellular organisms green and flagellated. This experiment allowed to isolate these green microorganisms. The microscopic observations of these cells showed characteristics of the micro-algae, showing (among other things) the presence of a violet color after treatment with iodine, revealing the presence of starch, diagnosis of photosynthetic activity. The contact between non-symbiotic juveniles, reared under sterile conditions, with these flagellate green cells allowed to induce photosymbiosis: these founding works demonstrated that the