190 NATURAL HISTORY. 



Siliquaria ; but for the most part spiral, though exhibiting an endless diversity in their proportions as 

 well as in their sculpturing and colour. 



All insects, crabs, and other articulate animals are symmetrical, having the organs in pairs, 

 i.e., the right side like the left. In Corals and Star-fishes this two-sidedness is usually disguised by a 

 radiate arrangement of the parts. But in. Snails the symmetry of the eyes, tentacles, and other organs 

 of the head is lost in the body of the creature. Instead of a double heart and two series of gills, 

 these organs are single, and placed on one side. When on the left side, the growth is from left to 

 right, to provide space, but in shells which are symmetrical, like the Pearly Nautilus, the Keyhole 

 Limpet, and the Ampullaria, the gills are developed symmetrically on each side, or nearly so. 



The tendency to grow in a spiral form is very characteristic of the class Mollusca. Some 

 writers have accounted for it in a very matter-of-fact way. " Molluscous animals are long and worm- 

 like, therefore Nature has coiled them up, that their tails may not be an incumbrance to them." It is 

 easily ascertained that the Snail has a small spiral shell when it first quits the egg, and the young 

 Whelk may be examined while still a prisoner in the same capsule with its brothers and sisters. The 

 convenience of the arrangement is obvious, and that may be sufficient for us at present, but the time 

 is coming when naturalists will desire to look more closely into these things. 



How happens it that the embryo Snail, coiling itself up closely in its narrow cell, almost always 

 takes a direction from left to right, following the course of the sun, and forming a dextral spiral, or 

 right-handed shell, like an ordinary screw 1 Such a course is not absolutely necessary, neither is it 

 accidental. A few Whelks and Garden Snails perhaps one in ten thousand are left-handed, 

 and certain kinds of Whelks and Land Snails are as frequently reversed as right. The greater part of 

 the genus Clausilia (numbering upwards of two hundred species) is reversed. The species of the 

 genera Physa and Triforis appear to be always reversed. All the specimens of Fusus contracts, 

 so abundant in the Red Crag, and also found living in Vigo Bay, on the coast of Spain, are 

 left-handed. But after all these latter are the exceptions. Every one familiar with garden plants 

 will have noticed that the hop turns round its pole in one direction, going to meet the light, while the 

 scarlet-runner takes an opposite course, as invariably as the sun it follows.* 



Shells owe their variations in form to a number of circumstances. Those which assume a spiral, 

 vary in being either turbiuated or discoidal in their growth, and again, in the infinite gradations 

 between the extremes of these two. The shape of Comes is an inverted triangle, that of the " telescope 

 shell " (Cerithium telescopiuni) is trapezoidal, and so on. The turbinated shells again merge into forms 

 in which the whorls become detached with age, as in Vermetus ; or a nearly straight tube, like Denta- 

 lium. The discoidal shells graduate into forms having fewer and fewer convolutions, and wider and 

 simpler mouths, until at last, in forms like the Limpet, all spirality is lost, and we have only a tent- 

 shaped cover. 



At almost the earliest period in which we discover evidence of the existence of man, we find the 

 primitive races dwelling upon the sea shore, and subsisting largely upon Mollusca ; leaving at one 

 point shell-mounds of oystei'-valves, associated with rudely-fashioned flint knives, employed in opening 

 them ; at another, the broken fragments of turbinated univalves, and the round stone hammers used 

 in crushing the shell to procure the bonne-bouche it enclosed. Nor did the mere cravings of hunger 

 impel them to seek shell-fish as articles of food, for in the limestone caverns of France and Belgium 

 numerous remains of shells of Mollusca have been met with, pierced with holes for the purpose of 

 attaching them to some article of dress or head-gear. 



Among the aborigines of the present day, in whatever region of the earth they dwell, the same 

 economic uses of Mollusca prevail, and their practices serve to throw much light upon the fragmentary 

 remains of their pre-historic ancestry. 



The second class of Mollusca, called Gasteropoda^ from the fact that the animals included in it 

 habitually creep or glide by the successive expansion and contraction of the under side of the body, 

 which forms a broad muscular foot is well exemplified by the common Garden Snail. If one of these 

 be watched through a window-pane in the act of creeping on the surface of the glass, the muscular 

 movements of the foot may be seen following one another in rapid wave-like rhythmical succession. 



* Unluckily, the botanists have chosen to reverse the terms employed in mechanics, and call the spiral of the hop right- 

 handed, t Caster, belly ; pous, foot. 



