322 MOLLUSCS. 



Dentalium, or a Carinaria and a Twrritetta, 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 

 500 Ibs. 



As every mollusc 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 011 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 mollusc 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 limpet ? 

 In very deep water the tendency to tenuity in the molluscan 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 molluscs 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 colours of shells are due to glands situated on the 

 Coloration. 



margin or the mantle. In most cases the colour markings are placed 



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

 inner layer of porcellaneous shells is of a different colour 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 colour 

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

 hedge-row snails, Helix nemoralis and H. hortensis. Here we find the ground- 

 colour 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 colour-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-colour, 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 colour hardly ever met with 

 in the shells of molluscs. One or two species of land-shells (Corasia) from the 

 Philippine Islands more nearly approach this tint than any other molluscs, but 

 even in these there is a slight admixture of green. On the contrary, blue is a 

 colour more commonly seen in the soft parts. The colour of the shell does 



