1882.] Transformations of Planorbis. 143 



cases and have lately found some diseased shells of PL trochi- 

 forvtis, Fig. 7, PI. vi. These are dwarfed, and show a tendency 

 to unwind the spiral ; so that they look remotely like the begin- 

 ning of a series of transition forms from PL trochiformis to PL 

 denudatns, Fig. 24, PI. vi. 



The following extracts contain the author's conclusions on the 

 influence of the environment on mollusks: 



Darwinists would say that want of bilateral symmetry or the 

 unsymmetrical spire was of advantage to the animal, therefore it 

 was selected and perpetuated. 



Now this statement may be readily accepted, with the under- 

 standing that the "therefore" does not imply a relation of causa- 



Most of the characteristics caused by the physical surroundings 

 are of course advantageous, but the physical forces are the causes 

 and not the advantageous or disadvantageous nature of the char- 



The form of the embryonal shell is straight, bag like or swol- 

 len, tubular ; Figs. 29-30, PI. VI. 



The simplest form of all is a disk with which this embryonal bag 

 begins, but this or the embryonal shell is not necessary for us to 

 consider. 



The coiled unsymmetrical shell (da) is carried above the 

 foot (k) as in the Helix pomatia, Fig. 8, PI. vii, and Planorbis, 

 Fig. 8 a PI. vii. This is built by the mantle or internal soft cover- 

 ing of the body d, Fig. 9. The shell has been removed, and the 

 fleshy cone of the mantle containing the stomach, intestines, etc., 

 has been partly unrolled to show that it was originally coiled up 

 'nside the shell. The structure of the shell can be more easily 

 understood in forms like Figs. 11, 12, PI. vii, where the mantle is 

 not so long and not coiled, but builds a broad, evenly balanced, 

 conical roof above the foot as in the Patella or Limpet. If this 

 shell be divided into halves and one half removed as in Fig. 12, 

 the structure of the shell becomes visible and also the relations of 

 the mantle d, and the mantle border d' , to each other and to the 

 two layers of the shell, e, and e' . 



This animal was once small enough to occupy only the upper 

 Part of this cone and then as it grew in size built the shell above 

 itself. The outer layers, e, Fig^, the outer edges of which are 

 seen also on the surface of the shell, da, Fig. 1 1 , were plastered up, 

 one inside of the other, by the mantle border, d+, which exactly fits 



