Symsagittifera roscoffensis, also called the Roscoff worm, the mint-sauce worm, or the shilly-shally worm, is a marine acoel worm. The origin and nature of the green color of this worm stimulated the intrigued zoologists in the 1870's. It was discovered that the coloring resulted from the symbiosis between the animal and a green micro-algae, the species Tetraselmis convolutae, hosted under its epidermis. It is the photosynthetic activity of the micro-algae in hospite (in the host's body) that provides the essential nutrients for the worm. This partnership is called photosymbiosis, from "photo", "light", and symbiosis "who lives with". These photosynthetic marine animals live in colonies (up to several million individuals) on the tidal zone.

Although roscoffensis means "who comes from Roscoff", this flatworm is not endemic to Roscoff or North Brittany. Its geographical distribution extends over the Atlantic coast of Europe; colonies were observed from Wales to the south of Portugal.

In 1879, at the Station Biologique de Roscoff founded by Henri de Lacaze-Duthiers, the British biologist Patrick Geddes pondered the nature and origin of the green compound of a local acoela he called Convoluta schultzii. He succinctly described "chlorophyll-containing cells" and the presence of associated starch "as in plant chlorophyll grains".

In 1886, French biologist Yves Delage published a detailed histological study describing (among other things) the nervous system and the sense organs of the same Roscoff acoela, Convoluta schultzii. In this article Delage also inquired as to "the nature of zoochlorella (i.e. micro-algae): are they real algae? Where do they come from? What are the symbiotic relationships that unite them to their commensal?"

In 1891, Ludwig von Graff, a German zoologist from the University of Graz and a specialist in acoela, undertook a taxonomic redescription of the Roscoff acoela at the Station Biologique de Roscoff. His works highlight a taxonomic confusion in the previous works of Geddes and Delage: "... the researches made in Roscoff and on the Adriatic coasts showed me that the green convoluta of the Adriatic (Convoluta schultzii) ... is specifically different from the green Convoluta of Roscoff. I have preserved for the first one the old name Convoluta schultzii, for the second I give the name of Convoluta roscoffensis." The species name roscoffensis is thus a tribute to the Station Biologique de Roscoff and Henri de Lacaze-Duthiers. In this article, von Graff also writes about zoochlorella (i.e. micro-algae) "Although they undoubtedly derive from algae ... they represent however, in their current state of adaptation, a specific tissue of the worm dedicated to assimilation".

It is in an academic book entitled Plant-Animals published in 1912 by Frederick Keeble, the co-discoverer of the nature and the origin of micro-algae, where a set of works is describing the specific relationship between Convoluta roscoffensis and its photosynthetic partners.

The 1960's brought with them a better understanding that called upon emerging electron microscopical techniques. Luigi Provasoli, a pioneering micro-algologist in the art of isolating and cultivating micro-algae, maintained for several years between 1965 and 1967 in New York (Haskins Laboratories, now in Connecticut) generations of S. roscoffensis directly received from the Station Biologique de Roscoff. Thus generations of non-symbiotic juveniles were born in New York and were used to test the specificity of association between S. roscoffensis and different species of micro-algae. A century after its formal description, in 1991, thanks to the techniques of molecular biology and the emerging tools of bioinformatics, the phylogeny of acoela was revisited and modified: the green Convoluta of Roscoff, Convoluta roscoffensis became Symsagittifera roscoffensis, the "Roscoff worm".

Symsagittifera roscoffensis lives in the tidal balancing zone, preferentially in a sandy substrate, poor in organic matter. The accumulation of the latter generates reduced, hypoxic or anoxic conditions, which promote the development of anaerobic bacteria releasing compounds such as hydrogen sulfide (H₂S) incompatible with the oxygen requirements of the worm.