Mollusca. 79 



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- 

 vinui are the following. 



The Ear-shell, Haliotis (Fig. 12G), 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. Siiitable pieces are sawni 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 u.sed 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 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. 21) is represented in the Aquarium by several species, 

 whicli 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 purple. The shade of violet produced, depended on 

 the amount of the dye used, so tliat 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 co- 

 louring 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 peojile as a signal. For tills purpose tiie 

 tip is cut oft", and the opening tiius formed is the moutii-piece of the 

 trumpet. The murmuring soimd, like that of the waves, which is heard 

 on holding this and other large shells to the ear, is very likely due to 

 the resoundiuii' 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. 2U), is the largest snail of tlie Mediteranean, 

 with a thin, rounded siiell. 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 

 Va per cent free hydrochloric acid. The animal squirts out considerable 



