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of their shells. They stick their eggs to the inner sides of their 

 house; when the larvae issue from the eggs, they swim about, 

 until, after a while, they also settle in one place. 



The Murex or purple-snails are represented by many kinds 

 (M. trunculus, brandaris), which belong to the commonest shells 

 of the Mediterranean , and in ancient times played an impor- 

 tant part. For from these mollusks was obtained the real pur- 

 ple used by the ancients to dye their royal garments. This pur- 

 ple substance is excreted from a gland lying in the animal' s 

 mantle, and, when freshly taken, is white or slightly yellow. 

 Placed in the sun it changes to lemon-colour and green, and 

 then to a beautiful violet, which becomes darker and darker the 

 longer it is exposed. The shade depends on the quantity of mat- 

 ter used, so that the dyer can obtain any tint he desires. The 

 making of this purple dye was a large branch of industry among 

 the ancients, and factories for the purpose were scattered all 

 over Italy and Greece. Monte Testaccio in Rome is formed of 

 the refuse from the greatest of these factories. Also in Aquileja 

 traces have been found of a purple-dye factory. In our days the 

 industry has entirely vanished, though the colouring matter , 

 so susceptible to light, is very well adapted, as experiments 

 have proved, for photographs on silk and other fine stuff. 



The Tritons-horn (Tritonium nodiferum) is a large plump 

 snail, with protruding head , feelers provided with eyes, and 

 a long elastic proboscis. This snail creeps slowly about at the 

 bottom of rather deep water and lives on animal food. Its 

 heavy, beautifully-twisted shell was used in ancient times as 

 a war-trumpet, and even now is employed as a signal-horn by 

 workmen (for example, the master-masons in Naples use it to 

 call their people together). For this purpose the point is cut 

 off and you-blow through the hole. The peculiar rushing sound 

 heard in holding a large shell to one's ear is produced by the 

 resonance of its hollow convoluted interior, which rejects the 

 waves of sound. In an absolutely quiet place the shell would 

 not sound at all. 



The barrel-snail (Dolium galea) is the largest in the Me- 

 diterranean, with a thin shell, shaped like a barrel, a white- 

 body spotted with dark brown, and a large proboscis. A pe- 

 culiarity of this animal is the shape of the salivary glands , 

 which attain considerable size (1 inches in diameter) , and 

 exude a fluid containing more than 2 per cent of free sulphuric 

 acid and nearly f of free muriatic acid. When defending itself, 

 the animal spurts out of its mouth a considerable quantity of 



