THE TEREDO AND ITS DEPREDATIONS. 



401 



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 role to a part of the mantle of the ani- 

 mal, extending like a fold to the margin of the shell, Corilland has in- 

 dicated 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 micro- 

 scopic 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 fast- 

 ened 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 per- 

 forate, 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 ad- 

 ductor muscles, which we will soon examine more closely. 



The shell presents, even when the valves are brought closely to- 

 gether, three large openings. 

 vol. xiii. 26 



