GENERAL DESCRIPTION OF MOLLUSCS. 611 



set of organs for preparing and digesting the food ; an excretory system ; and 

 reproductive organs. 



The shell in the majority of Mollusca consists of a single more or less 

 coiled tube (a.s in the whelk and snail), and hence they are termed univalves ; 

 in the oyster and other bivalves the shell is in two pieces ; whilst in the 

 coat-of-mail shells, chitons, there are eight little overlapping shelly plates. 

 The shell is composed mainly of carbonate of lime, with the admixture 

 of an organic substance known as conchiolin, which is very similar to chitin, 

 the horny material forming the hard parts of insects. It is generally 

 divisible into layers exhibiting differences of structure. The outermost, 

 called the periostracum, largely and often entirely consists of conchiolin ; it 

 varies greatly in appearance, being sometimes smooth and shiny, at others 

 rough and coarse, frequently it is fibrous. In many forms it readily rubs 

 off, in others it is firmly united to the true shell beneath. The term 

 epidermis, which has been misapplied to this layer, should be reserved 

 exclusively for the outermost layer of the skin of the animal itself. Of the 

 shell proper the main layer is usually either porcellaneous or crystalline in 

 structure, and in certain cases, such as Pinna, it is prismatic. The inner- 

 most layer is frequently pearly (nacreous), and in some, like the fresh-water 

 mussels or the pearl oyster, it is the thickest of all. The iridescent play of 

 colours, for which mother-of-pearl is noted, results not so much from any fine 

 microscopic sculpturing of the surface as from the breaking up and refraction 

 of the light that falls on them by the extremely thin films of which the 

 pearly layer is built up. Yet another quality of shelly matter is that which is 

 deposited when a damage in the shell has to be made good, or an unoccupied 

 space requires to be filled up. Very similar, too, is the calcareous material of 

 which the trap-door (operculum) closing the mouth of some shells is formed. 

 These different layers of shelly matter are secreted by a series of special 

 cells near the margin (collar) of the tough muscular skin (mantle) that 

 envelops the greater portion of the animal's body, the outermost layer 

 by the cells nearest the margin, the next one by those further in, 

 and so on. The shell, therefore, increases in size by the addition of 

 fresh material to its free edges, the whole being subsequently thickened 

 and strengthened by further deposits on the inner surface. The successive 

 additions along the growing edge generally leave ridges or marks parallel 

 with it, that are known as lines of growth. The deposition of shell does 

 not go on continuously ; every now and again there comes a period of 

 rest, and these rest-periods are frequently indicated by the occurrence of 

 a stronger ridge or mark. The different details of sculpturing on the 

 surfaces of the shell, striae, ribs, spines, etc., are all the products of corre- 

 sponding irregularities on the margin of the mantle, and were, when first 

 formed, situated on the growing edge. Pearls result from the coating over 

 with nacre of any irritating object that may have got between the mantle of 

 a pearl-producing mollusc and its shell a fact taken advantage of by the 

 ingenious Chinese who introduce small metal images. More often, perhaps 

 and certainly it is the case with the finer examples they result from a 

 diseased condition of the cells that secrete the nacre. 



The muscular foot, which is generally an organ of locomotion, takes various 

 forms in the different groups of Mollusca: the univalves (snails and whelks) 

 creep along by its means ; the bivalves employ it to burrow with ; in the 

 cuttle-fish it is drawn out into the arms ; in other Mollusca it is transformed 

 into fins to swim with ; whilst in some, like the oyster, it has ceased to be 



