Mollusca. 79 



the mantle, a flap of the skin, and is held on the animal by means of 

 a spiral muscle. The foot frequently carries a horny or calcareous lid, 

 which closes the mouth of the shell when the body of the animal is 

 retracted. The collections of shells in museums give some idea of 

 their beauty and brilliant colouring, as also of the great variety of their 

 forms; most are marine. The most important ones found in the Aqua- 

 rium are the following. 



The Ear-shell, Haliotis (Fig. 126), has a very flat ear-shaped shell 

 with a large opening, and a row of holes through which tentacle-like 

 processes of the foot protrude. Internally the shell is coated with the 

 most lovely mother-of-pearl, and is therefore frequently used for orna- 

 mental purposes. 



The Helmet-shell, Cassis (Fig. 22), is also commercially a very 

 important shell. The shells of the largest specimens are used in 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 condition; 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. 23) possesses in so high a degree the power 

 to take up water into the cavities of its foot, that it becomes 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. 21) 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 sun- 

 light, and finally brilliant purple. The shade of violet produced, depend- 

 ed on the amount of the dye used, so that the dyer was able to pro- 

 duce 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. 19), 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 and 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. 20), is the largest snail of the Mediteranean, 

 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 sulphuric acid and about 



