STANDEN : REVERSED SHELLS IN THE MANCHESTER MUSEUM. 229 



Obeying this law the mollusc winds its dwelling in a uniform direction 

 through the space around its axis. 



" Now it will be readily understood that, as the mantle of the 

 mollusc secretes the shelly integument, there is before the animal 

 an almost infinite choice as to the way in which it will build up 

 its home. So long as the form of the shell is regular, following out 

 the principle of a cone curved into a spiral, and descending in a 

 screw-like manner from the apex (or initial whorl) to the aperture, 

 the animal may select almost any variety of convolution. If un- 

 interfered with by any foreign obstruction, the animal, with unerring 

 certainty, will mould for itself a habitation, which, as I said, will be 

 finished with an absolutely perfect devotion to geometrical curves, 

 proportion, and principles. It is therefore a fascinating study to 

 observe how infinitely varied the series of curves may be, and how 

 wide is the scope granted to every mollusc in the erection of its home, 

 the only condition being that, in the case of the "regularly spiral" 

 shells, the law of ' the spire of the logarithm ' must be strictly 

 adhered to. 



" We observe then that, as the result of the unequality of growth in 

 the mantle and shell, there arise spiral twistings, and these eventually 

 produce an almost infinite diversity of curve. A series of torsional 

 convolutions may be traced, for example, from the long, many-whorled 

 Terebra, to the broad, flat, depressed Haliotis, and from that again 

 right on to Patella. Or again the regularly built-up Turritella may 

 be pulled out into the fantastic contortions of Ver/netns, and the un- 

 rolling may be carried so far that the whorls are all straightened out 

 into a single tube like Dentallum. Once more, when a shell such as 

 Cyprce.a or Conus is examined, it is seen that the body of it is made 

 up simply of the last whorl, with the ghosts, so to speak, of its pre- 

 decessors visible in small detail on what was once an elongated spire. 

 But whether the spiral convolutions are visible in the adult stage, or 

 only in the embryonic condition of the shell, the fact remains that the 

 twist has exercised an important function in the life history of the 

 mollusca. Prof. Sydney J. Hickson^ cites with approval Lang's 

 generalization on this subject.^ ' The formation of a spire-like shell, 

 which has been recognised as the starting point in the development 

 of the asymmetry of reptant Gastropods, was the only method by 

 which complete protection of the whole body could be attained, and 

 must therefore be considered to have been advantageous under the 

 circumstances.' 



" Now all molluscs with spiral shells must, naturally, twist either to 

 the right or to the left. And as a matter of fact, the vast majority of 



I "Torsion in lAo\\u.9,ca.," Journal 0/ Conc/iology, vol. 9, pp. 9-15, 1898. 

 H " Text-book of Cotnp.irative Anatomy," vol. 2, p. 150. 



