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abode here. That they also take part in their host’s meal is quite 
easy to assume, as Polyclads are readily fed with pieces of flesh. 
Thus I have watched Prosthiostomiim species feeding. That the two 
species in question have been firmly established is no wonder when 
the host provides cover, protection, and food. 
Are these small robbers of any advantage to the host? Pos- 
sibly as a kind of scavenger, for a hiint on small creatures and a 
consumption of decomposing animal stuff. Such a point of view 
would bring them in the category of commensalism, the term taken 
in its proper sense. The classical example of this is taken just from 
a Hermit Grab, Eiipagurus bernhardus with Åctinia adamsi. Being 
to a large extent limited to a theoretical discussion, the question 
thus cannot be definitively decided. But the case is too interesting 
to be passed by and, for the Polyclads, it has never previously been 
treated at any length. 
Summarizing I think we are on the safe side to assume that 
these Polyclads belong to the category of synoecism („Synoecie“ 
of German authors) and to exciude true parasitism. The damage 
done to the host through partaking of the food, devouring flesh 
fragments, can hardly bs regarded as serious and can consequently 
be neglected. Of quite another importance is undoubtedly a possible 
robbery of eggs and embryos. That the modus of life that these 
Polyclads have started may by degrees evolve into a true parasitism 
is not excluded. At least Euprosthiostomum is not without weapon, 
possessing a penis-stylet. In soft tissues such an organ inflicts 
wounds, as proved in many instances for the Cotyleans. For the 
present case I think it powerless, lacking sufficient strength to 
pierce the cuticle-covered tail-end of the host. Direct observations 
on the habits of these Polyclads would certainly be of interest and 
their ferocious robber-nature would then undoubtedly be proved. 
