Dictionary of Ichthyology  

inquiline = lodger; an animal that lives habitually in the cavity or abode of some other species, causing no harm, e.g. Nomeus in the Portuguese man-of-war, Physalia; Fierasfer in the pearl oyster, Meleagrina; Paraclinus marmoratus in finger sponges; Gilbertidia sigalutes in sponges, and Carapus in sea cucumbers. But see also endoparasite.
Fierasfer (= Carapus) from Lydekker et al. "Reptiles, Amphibia, Fishes and Lower Chordata", 1912. Photograph by Brian W. Coad.

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