THE TENDENCY TO SPIEAL GROWTH. 117 



not choose to leave them exposed to their foes in a fleshy mass, protected, 

 if at all, only by stinging or bitter qualities which might make them 

 noxious to the taste, such as are possessed by the unarmed hydroids and 

 actinians. The only alternative, apparently, is to give them a stout 

 armor. This is the service which the heavy shells perform in oyster-life. 

 That it is only partly effectual, a short acquaintance with the subject suf- 

 fices to show; a fact that seems to me to argue strongly against the va- 

 lidity of the view — one, I am glad to say, less prevalent now than formerly 

 — that all details of this kind were arranged fixedly from the start, and 

 must necessarily be considered the most perfect that could be devised. 



I have said that when the oyster attaches itself by the rounded surface 

 of one valve and grows in a sidewise position, as it is very likely to do in 

 regions where the bottom is disturbed by constant raking, the lower valve 

 soon becomes far stouter and broader than the upper; but at the same 

 time the general form is more symmetrical, rounded, and less inclined to 

 show a positive twist with advancing age than is observable where oys- 

 ters grow long and thin in the midst of dense clusters. Many foreign 

 relatives of this mollusk are far more irregular in form than ours. 



Oysters everywhere lie upon the left side ; a case where the right side 

 is underneath is very rare indeed. Why this is so it is perhaps impossi- 

 ble to say with certainty. Probably it is a manifestation of a tendency 

 observed throughout the whole branch of mollusks, where the coiled 

 (univalve) shells all twist from right to left, but where individual excep- 

 tions occur here and there, amounting in one case, among the fresh-water 

 physas of our brooks, to the reversal of the custom in a whole genus — 

 one, by-the-wa}', that is marked by extreme irregularity and variation 

 in form among its wide-spread members. 



It is this tendency, however it may have originated, to grow on one 

 side to the prejudice of the other, which seems to have produced coiled • 

 shells in the great variety of shapes we see; and naturalists believe that 

 they have all come from ancestral bivalved mollusks through a process 

 of natural selection acting primarily upon this supreme tendency. The 

 operculum, or little horny plate attached to the "foot" of a large num- 

 ber of mollusks, by which they stop up the circular aperture of their shells 

 when they retire within them, is considered the remnant (modified for a 

 new utility) of the aboriginal left valve, because the lines.of its accretion 

 show that in its growth it followed the same principle of the coil. In 

 some families, as the Limneidw, it has disappeared altogether ; but I think 

 that in nearly or quite every case where an operculum is no longer to be 

 found, the animal is minute, and the whole shell has become so thin and 



