AN ACETYLENIC ALLENE
Marasin is an unusual compound containing not only two acetylenic bonds (a diyne), but two contiguous double bonds (a cumulated diene, otherwise known as an allene). The simplest allene is
Allene itself, which is 1,2-propadiene. The next simplest is 1,2-butadiene (or methylallene, which is a flammable gas used as a feedstock in the manufacture of synthetic elastomers (artificial rubbers).
The two short allenes, Allene itself and 1,2-Butadiene, shown above are not present (as far as is reported) in the fungus, they are just for reference.
The diyne/allene grouping in Marasin arises due to the enzymatic re-arrangement of an alkyl-triyne moiety (which has three acetylenic bonds). Marasin exhibits anti-biotic activity.
It is somewhat strange that the same compound, Marasin, is also reported in the similarly named mushroom
Marasmiellus ravealis but your author thinks that there may have been some confusion over which mushroom contains it; with Marasmius ravealis seemingly getting many more internet hits than does Marasmiellus ravealis, the confusion possibly having arisen due to the ambiguous use of the abbreviations M. ravealis. Or it could be that both mushrooms manufacture it; they are, after all, both in the same family.