MOLLUSCS. 



79 



Fig. 75. — Section a of a shell (Pinna), and 6 of a pearl, the 

 former showing a prismatic structure, the latter the peculiar 

 laminated condition on which the iridescence depends. 



section of a clam-shell figured these points are illustrated diagrammati- 

 cally. Thus, in a way, these lines of growth correspond to the rings of 

 a tree, but it is impossible to correllate them with time — to declare the 

 age of a certain shell by counting its lines of growth. 



Shell has also its microscopical side, and according as the shell is 

 pearly, horny, porcellaneous, or the like, so its minute structure varies. 

 Our figure shows magnified sections of a horny shell (Pinna), and of a 

 pearl. The latter deserves a moment's attention, since it illustrates well 

 the feature to which pearls 

 and all iridescent shells owe 

 their peculiar beauty. As 

 will be seen, the pearl is 

 made up of a large number 

 of thin, undulating, but still 

 approximately parallel lines, 

 and it is a curious quality of 

 all such structures to break 

 up light and produce a spec- 

 trum, just as does a prism. 

 Indeed, the same means is used to-day in the physical laboratory in the 

 study of light, parallel lines being ruled very close together upon polished 

 steel or speculum metal, and these ' diffraction gratings ' produce spectra 

 better adapted for study than those caused by the prism. 



Shells vary greatly in shape, yet all except the Chitons may be grouped 

 in two categories, — the bivalves and the univalves. They also differ in 

 color, all colors, except those of iridescence, being pigments secreted by the 

 mantle. Though faster than most animal colors, those of shells are not 

 absolutely permanent. The shells which were once the treasure and pride 

 of Linnseus, long ago lost almost every trace of their color and markings. 



Molluscs reproduce solely by eggs. We do not find anywhere the 

 slightest trace of that budding or fission which is so common among 

 the worms, as also among the still lower types of animals. Yet it may 

 be said that the molluscs and the worms have many points in common, 

 and that the general structure of their common ancestor is' well known. 

 To-day we divide the molluscs into three great groups, — the Acephals, the 

 Gasteropods, and the Cephalopods. Of these, the gasteropods seem to 

 represent the central stem from which the others have diverged, and some 

 of the lower gasteropods, even in their adult condition, clearly show their 

 vermian affinities. 



