834 Marvels of the Universe 
water, but is too narrow to admit enemies. Again, just as barbed wire and broken glass are used 
by humans to prevent intrusion on private property, so the shells of some molluscs are provided with 
jagged and twisted teeth at the mouth, which serve the same purpose. Finally, there are the 
curious shells at the centre of the illustrations on page 830, which have these doorways completely 
inverted, so that the snails walk with the points of their spirals touching the ground. 
Only a brief mention can be made of the ornament of shells (see below). Often this ornament 
serves a protective purpose, as, for instance, the spines, which render their owners disagreeable to 
predatory marine life, for, if swallowed, they would cause the serious laceration of an unwary gullet. 
Such molluscs, therefore, as the Thorny Oyster and Thorny Woodcock are left undisturbed, to live 
out their lease of life. All ornament was probably developed from ridges. These ridges first took 
the form of corrugations, then became more pronounced and of the nature of frills ; while the frills 
in time became elaborately goffered, until the corrugations closed together and settled definitely into 
spines. It is usually the margin of a shell which is frilled, and when the creature adds a later 
margin, the frill remains as a crest or a ridge of thorns on the exterior of the shell. 
Of coloration we have here little space to write—and who can describe the indescribable ? When 
it is remembered that most of this colour is hidden by masses of weeds and living matter, which 
becomes attached to a shell at the bottom of the sea, it is difficult to determine the purpose of such 
elaborate beauty ; and it is equally difficult to conceive Nature to be so prodigal that she lavishes 
the beauty on a mollusc for a mere whim. Sometimes, it is true, coloration serves the purpose of 
protective resemblance and mimicry, but such cases are rare, and do not include the dazzling reds 
and purples and golds, or the pastel shades and peacock colourings which distinguish so many 
varieties of shell-life. 
THE ORNAMENTS OF SHELLS. 
They can be roughly classed as “‘frills’’ and thorns. Colour, which is the chiefest characteristic, cannot be shown in 
photographs; but the coloured plate gives a few examples of this most beautiful form of decoration. The shells on this plate are: 
(1) the Endive Shell, (2) the Scorpion Shell, (3) the Wentletrap, (4) the Thorny Woodcock, (5 and 6) the Thorny Oyster, closed 
and open, 
