MoUusca. 



Q3 



the cutting of the well-known cameos. Suitable pieces are sawn 

 out of the shell and their outer surface ground down, while the 

 inner surface remains in its natural conditions ; the relief is worked 

 out of the many-coloured layers by means of a graving tool such 

 as is used for cutting corals. 



The Snail Natica (fig. 22) possesses to so high a degree the 

 power to take up water into the cavities of its foot, that it can 

 become three times as large as it is in the normal condition. It 

 can also crawl at a fast pace, contradicting the proverbial slowness 

 and laziness of its group. 



Murex (fig. 23) is represented in the Aquarium by several 

 species, which played an important part in former days, furnishing 

 the Tyrian purple which was used by the ancients for their festal 

 garments. The colouring juice, the secretion of a gland of these 

 snails, is white or pale yellow when fresh, but turns yellow and 

 green when exposed to sunlight, and finally purple. The shade 

 of violet produced depended on the amount of the dye used, so 

 that the dyer was able to produce any tint. In ancient times this 

 dyeing industry was practised all over Italy and Greece. Now- 

 a-days it has entirely disappeared, although this colouring matter 

 has been proved so sensitive to light as to be well suited for 

 printing photographs on silk. 



The "Triton's horn", Tritonium (fig. 17), is a large, clumsy 

 snail with long tentacles and a long extensible proboscis. It 

 crawls about slowly at the bottom of the sea at considerable depths, 

 and lives on animal food. The heavy shell was used by the Romans 

 as a martial horn, and is still used by the people as a signal. For 

 this purpose the tip is cut off, and the opening thus formed is the 

 mouth-piece of the trumpet. The murmuring sound, like that of 

 the waves, which is heard on holding this or other large shells 

 to the ear, is very likely due to the resounding within the coiled 

 shell of the numerous waves of sound always present. In quite 

 still places no noise is heard in such shells. 



The Tun, Dolium (fig. 16), is the largest snail of the IMediter- 

 ranean, with a thin, rounded shell. Its body has dark brown spots 

 on a white ground and is provided with a large proboscis. A 

 curious fact in connection with this animal is that its large salivary 

 glands secrete a fluid which contains over 3 per cent of free sul- 

 phuric acid and about 1/2 per cent free hydrochloric acid. The 

 animal squirts out considerable amounts of this acid fluid in order 

 to soften the hard calcareous skin of the Echinoderms which it 

 eats, and perhaps also in self-defence. It is still a puzzle how these 

 strong corrosive acids are produced by the animal and how they 

 can be stored up. For some years past Dolium has become more 

 and more rare in the Gulf. 



An interesting animal is the Worm-shell, Vermetus (fig. 123), 

 from the fact that it is not able to move about freely like the 



