3290 THE MOLLUSKS OR SHELLFISH 



or a Carinaria and a Turritella, and among the bivalves between a Pholas and the 

 hammer oyster (Malleus}. The difference in size is still more remarkable, some 

 microscopic forms weighing not more than the fiftieth part of a grain, whereas the 

 gigantic Tridacna occasionally turns the scale at over five hundred pounds. 



As every mollusk has a history of its own, a certain course to pursue in the 

 living world, its means of existence and propagation will be found sufficient, 

 although to the human eye the chances against it may superficially seem over- 

 whelming. The glassy Carinaria, regarded as a dweller on the surface of the 

 ocean, would seem ill-adapted to withstand the buffeting of storms at sea; and so 

 it is, but there is very little doubt instinct has taught the mollusk the proper 

 moment, and the sufficient depth to descend from the surface, to be safe from the 

 tempest's violence. The dweller on the seashore, open to the onslaught of the same 

 unpitying foe, defends himself with a stronger abode. What dash of the waves 

 upon the undefended rock could in the slightest affect the conical shell of the lim- 

 pet ? In very deep water the tendency to tenuity in the molluskan shell is plainly 

 apparent, yet there are species dwelling at depths beyond the influence of surface 

 storms, the shells of which present considerable strength and solidity. We may 

 ask how and why is this ? Although we may not in all cases be able to answer 

 these queries from actual knowledge, certain is it, that reasons, probably very near 

 the truth, are easy of suggestion. For example, the large Cyprina islandica and 

 the heart cockle (Isocardia cor) of the British seas possess shells of considerable 

 strength and solidity. These, being mollusks of large size, would prove dainty 

 morsels to the hungry haddock or other fish, if they were unprotected by a power- 

 ful shell and their extermination thus prevented. 



The varied colors of shells are due to glands situated on the 

 margin of the mantle. In most cases the color markings are placed 

 on the outer surface of the shell, beneath the periostracutn, but occasionally the 

 inner layer of porcellaneous shells is of a different color to the outer. This is well 

 instanced in the helmet shells ( Cassis) , which are employed by the carvers of shell 

 cameos to produce white or rose-tinted sculptures upon a dark ground. The color 

 in some shells is liable to extreme variation. Take, for example, the common 

 hedgerow snails, Helix nemoralis and H. hortensis. Here we find the ground 

 color yellow, brown, pink, white, lilac, and various intermediate shades, and the 

 bands which are usually brown, and normally five in number, may be altogether 

 absent or vary from one to six, their position also being equally variable. This 

 diversity in color markings results from the different position of the pigment glands 

 upon the mantle margin, but the cause of this variation in the position has not been 

 ascertained, although it may be presumed; nor is the reason known of the difference 

 of the ground color, which may occur among specimens of the same colony. White, 

 black, red, green, yellow, olive, purple, slate-blue, and brown form the common 

 ground tints of shells, but pure blue is a color hardly ever met with in the shells of 

 mollusks. One or two species of land shells (Corasia) from the Philippine islands 

 more nearly approach this tint than any other mollusks, but even in these there is 

 a slight admixture of green. On the contrary, blue is a color more commonly seen 

 in the soft parts. The color of the shell does not necessarily correspond with that 



