i86 THE POPULAR SCIENCE MONTHLY, 



cockle or mussel. But for the toughened surface representing the 

 cockle's foot, we might regard tlie oyster as a lapsed form of some 

 ancient Cephalopod. 



The mantle secretes the shell, and in all bivalves it lies through its 

 whole extent against the shell. Now, in all moUusks, the axis of the 

 body is at first straight, and the body is bisymmetrical. If growth 

 were arrested at an early stage, all mollusks would look alike, and, if 

 the embryotic mantle were to secrete a shell, all these arrested growths 

 would appear as miniature bivalves. They would be symmetrical. 

 But circumstances determine shapes. The mollusk which, in maturity 

 as well as infancy, lives in the open sea, will be exposed to like condi- 

 tions on either side, and Avill retain its bilateral symmetry, A mollusk 

 which lies on the sea-bottom will be exposed to unlike conditions, one 

 side being buried in mud and the other bathed in water. As a shrub 

 which grows against a wall loses its symmetry and becomes one-sided, 

 so a young oyster, as soon as it leaves off its roving ways, and fixes 

 its abode on the sea-mud, must begin to develop unsymmetrically. 

 One side and one valve of the shell outgrow the other side and valve. 

 In the Gryphgea, an ancient sjjecies of oyster, this over-development of 

 one side is carried further, and, while the right valve is small and flat, 

 the left is deep and partially rolled up. In the Gasteropods, except 

 Chiton, this one-sidedness is cai*ried still further. One side outgrows 

 the other so much that the body takes a spiral form, and one valve, 

 secreted by one fold of the mantle, appears as a spiral shell, while the 

 other valve, secreted by the aborted fold of the mantle, appears as an 

 operculum a little shelly disk known under the name of "eye-stone," 

 In the snail this one-sided development is carried to the highest pitch 

 of asymmetry. Overgrowth of the right side forces it into a spiral, 

 and the right valve twists around with the body it incloses, while the 

 left valve, which, in the marine Gasteropod, we had found reduced to 

 an operculum, is here completely eliminated. 



From the cloaked clavelina to the oyster, we were led, stej) by 

 step, along successive modifications of the mantle. From the oyster 

 to the snail we have passed, step by step, along successive stages of 

 one-sided over-development. The facts have shown that a bivalve mol- 

 lusk could not have descended from a univalve. As all mollusks in 

 early lifQ have the axis of the body straight, and the parts symmetri- 

 cally arranged on either side, we may infer that bilateral symmetry 

 characterized the remote ancestors of the molluscan type. Now, 

 while a mollusk is bisymmetrical or nearly so, if the mantle secretes a 

 shell it must be in in two parts, or, as in Chiton, in many parts. The 

 snail is the last term of our series, and its successive stages of growth 

 should indicate the path along which Nature has moved in the evolu- 

 tion of the unsymmetrical Gasteropod from a symmetrical, oyster- 

 like bivalve (Fig. 4), 



Lereboullet has made out the embryology of Linmeus, a fresh- 



