1 882.] Transformations of Planorbis. 449 



In this case in which also the upper valve is the larger, it is this 

 which receives the direct support of the attached plug. 



The young are at first free, then attached by a byssus. Figs. 

 26, 27, show the right and left valves at an age when the animal 

 has fallen over on the right side, and the notch, *Fig. 27, begins to 

 be formed. 



Fig. 28 shows how the lower valve continues to build out around 

 the notch towards the anterior end, and in Figs. 29 and 30, it 

 becomes complete. This greater demand upon functional activity 

 of the lower side of the mantle, and the fact that this is the mova- 

 ble valve, explains why it is not larger than the upper valve as in 

 the oyster. 



As was pointed out to me by Mr. J. S. Kingsley,the growth of 

 the upper and lower valves sidewise is really an effort on the part 

 of the animal to recover, by lateral growth in a new direction, the 1 

 symmetry lost when it fell over on its side. Any one comparing 

 Figs. 28 and 31-30 will see that the long axis of the form in Fig. 

 31 is at right angl.es to what it is in Fig. 27. 



I shall call this tendency, to equalize the form in the direction 

 of a horizontal plane, geomalic ; the downward tendency of the 

 growth being designated by botanists, geotropic. 



The Anomia, when it falls over, loses its bilateral symmetry, 

 because the right and left sides become upper and lower, and being 

 in the vertical position, they are unequally affected by gravity and 

 by change of function.' At the same time the dorsal and ventral 

 sides, which were before vertical, have become horizontal ; and 

 the geomalic growth of the ventral side, in order to restore the 

 lost equilibrium in a horizontal direction, at once begins. This 

 does not attain perfect lateral symmetry. The form of the animal 

 cannot be easily changed, and the dorsal and ventral sides are 

 still distinguishable; therefore, it is necessary to call this tendency 

 of the growth by a new name, so that we can speak of the dorsal 

 and ventral as well as the right and left sides as having the same 

 tendency to assume by geomalic growth the natural and inevita- 

 ble condition of equilibrium. The experienced observer will at 

 once think of many apparent violations of this law. 



Many larval forms of Gasteropods begin to build the spiral, while 

 they are still within the egg, or still free swimming animals. 



Balfour 1 states, in his masterly summary of Comparative Embry- 



1 Comparative Embryology, vol. I, p. 190. 



