MOLLi:^C^. 249 



distinctive of most mollascs. In it one can frequently fin<l three distinct portions in 

 serial order, known respectively as the propodium, (in front; mesopodium, and nietapo- 

 dium. Occasionally lateral portions, epipodia, are developed. From the dorsal |)or- 

 tion of the body ari-ses 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 deser^-es 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 inter- y&i ^—= <^ 

 stitial growth. Such being the ca.se, it is an interesting question ^^^^§ . 

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



bum a bit of some shell like that of the clam, to dc-stroy the ani- , . 



mal matter, and then break it across from the hinge to the martnn 

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

 layers, each of which, as we proceed inward, Ls 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 before, this layer extends beyond the prece<ling 

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

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



surface is marked with parallel lines, the edges of the successive Ij 



layers. 11 



The structure of the shell presents many interestinsj ix)ints. vva. i&s. — i>iagraro- 



^ ' ^ ' mat!- '^rtior. "f 'hflL 



It may be hard and opaque, like porcelain, fibrous, glassy, homy, sb.>- j • - ■ -^m 

 or pearly, or nacreous, giving beautiful iridescent colors. On r' -a. 



microscopic examination it is seen that these latter owe their hues ci.1 , . . ^^-ive 

 to minute undulations of the layers, and that they are diffraction Sf'tuS addoctor'n^ 

 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 homy external layer, the so-called epidermis, 

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

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

 injury. 



At some st.ige 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 nearlj- 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 b 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 



