Tank No. 3. 
53 
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. 85, Tank 3) 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 brilliant 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. Nowadays 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. 
Fig. 86. Tritonium nodiferurn , J /3 nat. size. 
The "Triton s horn”, Tritonium (Fig. 86, Tank 3), 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. 87 Tank 3) is the largest snail of the Medi- 
teranean, 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 y 2 P er cen f 
free hydrochloric acid. The animal squirts out considerable amounts of 
