which is nothing other than a matrix synthesized from secreted mucus. By observing closely, we see that animals do not slide directly on the sand but "give the impression of sliding on an invisible surface". The author of this study hypothesized that animals, moving "above" the sandy substrate, would also receive more light by reflecting light rays, taking advantage of more photosynthesis and hence nutrient transfer for the worm. Secreted mucus is also an interface (biofilm) between the animal and its environment. Specific bacterial populations develop and appear to be hosted in the mucus, being hence intimately involved in the worm biology. This triptych (animal + micro-algae + bacterial consortium) is a good example of the Holobiont paradigm that explains that an organism (animal or plant) is a complex and dynamic association involving microbial populations necessary for development, growth and to some extent, for the life of the organism. S. roscoffensis does not have a digestive system at any time during its development. However, on its ventral side there is an orifice considered as "a mouth" which allows it to ingest (without digesting) the micro-algae Tetraselmis convolutae. The latter gives access to a digestive syncytium through which the future micro-algae in hospite is first vacuolated (it loses its flagella and its theca) then transits to be finally located under the epidermis of the animal without being internalized in the cells but in contact and between the cells of the worm. Symsagittifera roscoffensis also has a muscular system consisting of a complex network of transversal, longitudinal, circular and dorsal-ventral muscle fibers. This worm is hermaphrodite but does not self-fertilize: it must mate with a partner to reproduce. Mature spermatozoa are produced at the posterior end of the animal. The oocytes are fertilized by the spermatozoa of the partners who are stored, after mating, in a spermatheca. This spermatheca is connected to the oocytes by one canal. Each gravid individual will produce from the mucus that it secretes abundantly a transparent cocoon in which are released the fertilized oocytes. Embryos, whose number varies and can reach twenty, develop synchronously. In the laboratory and depending on the rearing conditions, after 4 to 5 days, the juveniles hatch, escape from the cocoon and begin their quest for the photosynthetic partner. If the micro-algae is not ingested, the non-symbiotic juveniles in the laboratory die after about 20 days. Phylogeny: incertae sedis? In 1886, Yves Delage