This animal is very common upon old walls and under stones. It is
somewhat like the wood-louse, but more flat; it is essentially a
dweller on the land, but it cannot exist except in damp places, where
the moisture is sufficient to keep its branchiæ pliable; it belongs to
a group of small Crustacea known by the name _Oniscus_. Some, as we see
in the present instance, frequent the land, but the greater portion
inhabit the water.
[Illustration: _Cloportus ascellus._
(Fig. 1, much magnified; fig. 2, natural size.)]
Among those which inhabit the latter element, there is a minute species
which is very injurious to timber. It excavates a cylindrical hole for
its dwelling, and increases in number so rapidly, that in a few years
timber which is covered with water is rendered useless. The temporary
wood-work used during the time the Bell-Rock Lighthouse was in the
course of erection, was destroyed, to a great extent, by this little
creature. When the wood had been under water for three years, beams ten
inches square were reduced to seven inches; at the rate of one inch
a year. Another species, _Cymothoa_, attaches itself to the backs of
different species of fishes, living upon the juices of their body.
A crustaceous animal nearly allied to this last is described in the
fifth volume of the _American Philosophical Transactions_; it is
accompanied by engravings which we have copied, but the animal is
not drawn with sufficient accuracy to be referable to any particular
species; by this account it appears that, instead of attaching itself
to the body of the fish, the parasite makes safe its lodgement on the
roof of the mouth. The author thus describes it.
[Illustration: _Head of Alewife._