cal means only. The manner in which the teredos proceed in their work has not, however, been clearly pointed out. In fact, while Hancock does not consider the shell, but the fleshy foot, as the boring instrument, and Quatrefages attributes that rôle to a part of the mantle of the animal, extending like a fold to the margin of the shell, Corilland has indicated the shell itself as the perforating instrument. By fastening the shell of a teredo on the end of a small stick of wood with gum, and turning it between the thumb and finger, he has succeeded, after four and a half hours' labor, in boring a hole in wood thirty millimetres deep. Mr. Harting arrived at the same conclusions by a careful microscopic examination of the shell and the muscular system of the teredo. We will point out the principal results of his studies, with illustrations to make them clear:
The shell is composed of two valves of equal size, which are not fastened together with a hinge; this is also the case with all other species of the genus Teredo and Pholas. The valves are maintained in place by a fold of the mantle in form of an arc (Fig. 1, b), which encircles them posteriorly. Moreover, the posterior part of the mantle has a
|Fig. 1.||Fig. 2.|
prolongation (Fig. 1, a, and Fig. 2, a) which covers, to a certain extent, the dorsal side of the valves, and extends on each side to their margin, forming two lobes (Fig. 1, c, and Fig. 2, b), which nevertheless do not adhere to the shell; by this mode of union the relative position of the valves is maintained. With other bivalve mollusks, which do not perforate, this relation is firmly fixed by a hinge; but, with the teredo, the valves have a certain play, which allows a slight displacement in their relative positions. The valves are, moreover, connected by two adductor muscles, which we will soon examine more closely.
The shell presents, even when the valves are brought closely together, three large openings.