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.
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.
Head of Alewife.
Part of lower jaw removed to show the insect.