the
pearly inner surface of the valves and projecting therefrom. The
flatter or less pronounced form of these nacreous excrescences are
often called " blister pearls," because of their resemblance to
vesicular eruptions, or water-blisters caused by burns. These
protuberant or vesicular excrescences, as the case may be, are induced
in two ways. First, and perhaps more commonly, by the perforation of
the shell, from outside to'inside, by some species of boring parasite,
pholads and lithophagi among the bivalve mollusks (Acephala), also by
certain sponges (Clione) and boring worms. For the most part, these are
not really parasites, as they do not derive their nutriment from the
substance of the pearl oyster, as leeches and ticks do from the blood
of their victims ; the term "domiciliaire"1 gives a clearer
idea of the relation of these forms to that upon which they fasten or
to which they attach themselves. These mollusks, sponges, and worms
simply make their residence or domicile, according to their habit, upon
or in the shell of the pearl oysters.3 The boring species
are quite small during the early adolescent stage when they first
attach themselves, but with increasing growth they have necessarily to
increase the size of their burrows, until at last, to the great
inconvenience and annoyance of the pearl oyster, the tunnelers have
pierced through its shell, and the oyster, in order to maintain the
privacy of its own domicile, is forced, as it were, to plaster over the
holes with a coating of nacre. This process is repeated and continued
as long as the tunneling goes on, until finally a
1 Name given by Robert E. C. Stearns in paper cited.
3
In addition to the particular species of fish, Fierasfer dubius,
figured in the plate, the occurrence of which had previously been made
known, Dr. Stearns has detected another, apparently belonging to the
Oligocottus, a form quite different from Fierasfer. The latter is a
long, slender, eel-like form, while the other is a shorter, chunky
fish, with a squarish head and rather prominent though stumpy spines.
The Oligocottse are small, bull-headed fishes that " usually inhabit
rock pools between tide-marks," and are peculiar to the North Pacific
waters. The Fierasfers inhabit tropical or semi-tropical regions, and
have been reported from Florida Keys to Cuba and Panama. The specimen
illustrated was probably from the Gulf of California, as well as the
Oligocottus, the occurrence of which as a parasite or domiciliaire had
not before been made known. (See Colored Plate No. 8.) It is highly
probable that still other species of the ichthyological section of the
animal kingdom may yet be discovered occurring under similar
conditions, for it would seem that small fishes of many species might
occasionally be chased into the gaping valves of the oysters when
pursued by some predaceous member of the finny tribe. The Fierasfers,
however, exhibit the parasitic habit, as has been pretty well
ascertained, not only through its occurrence in the pearl oysters, as
before shown, but also through similar relations to the Echinoderms.
