Mollnsca. 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 Aquarium 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 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, 

 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 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. Xow-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 murmur- 

 ing 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 alway present. In quite 

 still places no noise is heard in such shells. 



The Tun, Dolium (Fig. 20j, is the largest snail of the Mediterranean, 

 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 con- 

 nection with this animal is that its large salivary glands secrete a fluid 

 which contains over 3 per cent of free sulphuric acid and about i/ 2 per 

 cent free hydrochloric acid. The animal squirts out considerable amounts 



