THE ANATOMY OF THE RIVER-MUSSEL. 
241 
On the principle of “ Ab uno disce omnes,” the reader, it is 
hoped, will, after witnessing the dissection of a river-mussel, 
which I propose to perform in his presence, be in a position to 
appreciate the likenesses and unlikenesses which certain other 
members of the Lamellibranchiata have to one another, and to 
the representative which has been chosen to bear the investi- 
gation of the scalpel. 
The Shell. — It will first be noted that the animal is the 
tenant of a shell — and not the shell the occupant of the 
animal, as in the cuttle-fish — consisting of two irregularly 
oval folding-doors or “valves,” of like size and shape, con- 
nected together along one margin by a sort of hinge. It is, 
in fact, what conchologists term a “ bivalve.” 
It will naturally be asked which is the right and which the 
left valve of the shell, and which the anterior and which the 
posterior aspect of a given valve. 
On each valve may be seen a slight prominence — the 
“ umbo ” * — nearly surrounded by numerous concentric lines, 
and lying close to the edge which is hinged on to the neigh- 
bouring valve, and much nearer to one than to the other of its 
extremities. 
If, then, the mussel be held with the hinge-margin upper- 
most, and the umbo away from the observer, that end will be 
anterior which is farthest from him, and that valve, the right 
valve, which is on his right hand, and vice versa ; the dorsal 
edge of either valve being that by which it is joined to its 
fellow, while the opposite unattached edge is the ventral 
margin. 
It may be not amiss to state here that the bivalve shell of 
the Brachiopoda,f e.g. Terebratula and Lingula , does not 
answer to the lamellibranchiate shell, for its valves are dorsal 
and ventral instead of being lateral. 
The shell is strictly a structure secreted by the “mantle” 
(m m' fig. 1), that fleshy robe which invests the animal; being, 
so to speak, a hoarded, but not useless, lumber of cast-off 
clothing. Destitute of blood-vessels, it has no inherent power 
of growth or repair, but is dependent upon the mantle for any 
addition to its substance. 
The secretion of the true shell, with its covering of epi- 
dermis, is carried on by that thickest portion of the mantle 
which constitutes its margin or “ collar ” (see lower part of 
left lobe m', fig. 1), while by the thinner part (to which the 
letter m' points) is formed the “ nacre ” or pearly lining. This 
* Lat. umbo = the boss of a shield. 
t Often called “lamp-shells,” from their resemblince to an ancient 
lamp. 
VOL. IX. — NO. XXXVI. R 
