were captured was the following.  A long rope was let down into the sea, with baskets of reeds or rushes attached to it at intervals, constructed like our lobster-traps or eel-baskets, with an opening that yielded easily to pressure from the outside, but resisted pressure from the inside, and made escape, when once the trap was entered, impossible.  The baskets were baited with mussels or frogs, both of which had great attractions for the Purpurae, and were seized and devoured with avidity.  At the upper end of the rope was attached to a large piece of cork, which, even when the baskets were full, could not be drawn under water.  It was usual to set the traps in the evening, and after waiting a night, or sometimes a night and a day, to draw them up to the surface, when they were generally found to be full of the coveted shell-fish.[815]

There were two ways in which the dye was obtained from the molluscs.  Sometimes a hole was broken in the side of the shell, and the fish taken out entire.[816] The sac containing the colouring matter, which is a sort of vein, beginning at the head of the animal, and following the tortuous line of the body as it twists through the spiral shell,[817] was then carefully extracted, either while the mollusc was still alive, or as soon as possible after death, as otherwise the quality of the dye was impaired.  This plan was pursued more especially with the larger species of Purpurae, where the sac attained a certain size; while with a smaller kinds a different method was followed.  In their case no attempt was made to extract the sac, but the entire fish was crushed, together with its shell, and after salt had been added in the proportion of twenty ounces to a hundred pounds of the pulp, three days were allowed for maceration; heat was then applied, and when, by repeated skimming, the coarse particles had been removed, the dye was left in a liquid state at the bottom.  It was necessary that the vessel in which this final process took place should be of lead, and not of bronze or iron, since those metals gave the dye a disagreeable tinge.[818]

The colouring matter contained in the sac of the Purpurae is a liquid of a creamy consistency, and of a yellowish-white hue.  On extraction, it is at first decidedly yellow; then after a little time it becomes green; and, finally, it settles into some shade of violet or purple.  Chemical analysis has shown that in the case of the Murex trunculus the liquid is composed of two elementary substances, one being cyanic acid, which is of a blue or azure colour, and the other being purpuric oxide, which is a bright red.[819] In the case of the Murex brandaris one element only has been found:  it is an oxide, which has received the name of oxyde tyrien.[820] No naturalist has as yet discovered what purpose the liquid serves in the economy, or in the preservation, of the animal; it is certainly not exuded, as sepia is by the cuttle-fish, to cloud the water in the neighbourhood, and enable the creature to conceal itself.