MOLLUSCS. 



249 



distinctive of most molluscs. In it one can frequently find three distinct portions in 

 serial order, known respectively as the propodium, (in front) mesopodium, and metajjo- 

 dium. Occasionally lateral portions, epipodia, are developed. From the dorsal por- 

 tion of the body arises a fold of the body wall, the pallium, or mantle, which partially 

 or completely envelops the body. In some the two halves of the mantle may be dis- 

 tinct, while in others they are connected. This mantle plays no inconsiderable part 

 in the economy of the animal, for from it is developed the shell so characteristic of 

 most molluscs, and which deserves more than a passing mention. 



The shell is largely composed of carbonate of lime, together with more or, less animal 

 matter, the whole being secreted by the outer layer of the mantle. This shell is 

 entirely without blood-vessels, and is absolutely incapable of intei- 

 stitial growth. Such being the case, it is an interesting question 

 to decide how it increases in size. This is readily settled if we 

 burn a bit of some shell like that of the clam, to destroy the ani- 

 nial matter, and then break it across from the hinge to the margin 

 It will then be found that the shell is built up of a series of 

 layers, each of which, as we proceed inward, is larger than its pre- 

 decessor. The way in which the shell is formed by the mantle 

 explains this structure. When the animal is very small it secretes 

 a layer on the underside of the embryonic shell. With an increase 

 of growth another layer is laid down, but since the mantle is now 

 larger than it was befoi'e, this layer extends beyond the preceding 

 one. Other similar depositions follow, the result being that the 

 shell is thicker at the hinge than at the edge, while the outer 

 sui'face is marked with parallel lines, the edges of the successive 

 layers. 



The structure of the shell presents many interesting j)oints. 

 It may be hard and opaque, like porcelain, fibrous, glassy, horny, 

 or pearly, or nacreous, giving beautiful iridescent colors. On 

 microscopic examination it is seen that these latter owe their hues 

 to miimte undulations of the layers, and that they are diffraction 

 spectra similar to those now produced for physical researches by 

 fine rulings. The external color of shells is due to pigment deposited by the edge of 

 the mantle, which frequently bears the same pattern of ornamentation as does the 

 shell. Usually shells are covered with a horny external layer, the so-called epidermis, 

 which is likewise a product of the edge of the mantle. Its purpose is to protect the 

 shell from the corroding power of the water in which they live, or from other external 

 injury. 



At some stage of growth almost all molluscs bear a shell, but with some it disap- 

 pears with growth. The shell may be univalve or bivalve, or in the case of that aber- 

 rant group, the chitons, it may be composed of eight pieces serially arranged. In the 

 first case it is usually coiled in a spiral, although a conical form is not rare. Among 

 the bivalves the two halves of the shell are nearly alike, though in some the similarity 

 is largely lost. 



Turning now to the internal structure, we have first to take up the digestive tract. 

 This is always separated from the body cavity by proper walls. It begins with a 

 median mouth at the anterior end of the body, and terminates at the anus, which is 

 also primitively in the median line at the posterior end of the animal. The torsion 



i'XG. 254. ^ Diagram- 

 matic section of shell, 

 showing the method 

 of increase in thick- 

 ness during growth. 

 The black spots indi- 

 cate the successive 

 points of attachment 

 of the adductor mus- 

 cle. 



