GENERAL CHARACTERS. 
323 
not necessarily correspond with that of the mollusc. The latter may be of 
an intense black, the shell being quite white; the “animal” may be a most 
brilliant creature with a variety of many colours, and its test merely of some 
uniform sombre hue. Very gorgeously painted shells are, however, generally 
indicative of highly coloured inhabitants. 
That light and warmth are great factors in the production of brilliant colour¬ 
ing is beyond question. This is conclusively proved by the results of deep-sea 
dredging. Whenever great depths are reached, where darkness reigns, and the 
water is intensely cold, the molluscan inhabitants are without colour, or of very 
sober tints, although the periostracum, generally greenish or olivaceous, is scarcely 
modified under these circumstances. 
Our information with regard to the duration of life in molluscs is 
Age. . . . 
very limited. Although certain species of land-shells have been kept 
alive in confinement for four or five years, or even longer, we cannot assert that 
they exist for so long a period under natural conditions. The length of life of 
individuals, as well as of different species, is probably very variable, some doubtless 
attaining to a good old age. Such a species as the giant clam ( Tridacna ), for 
example, it is rational to conclude must have a very lengthy term of existence, for, 
although the growth may be rapid, the formation of a shell weighing 300 or 
400 lbs. must surely be the work of years; and, moreover, when the process of 
growth is ended, we know not for what period the shell may continue to live. It 
is likely that most land-molluscs are full grown in a year or two, but the term 
of their existence, after this, is probably very variable, according to the species. A 
specimen of the common periwinkle has been kept in an aquarium for nine years, 
but this scarcely indicates the limit of life of this species under natural conditions. 
Its average duration may be longer, but probably shorter. In connection with 
the length of life of molluscs, mention should be made of the long periods some of 
the species are capable of existing without food in a state of torpidity. 
In cold climates land-snails bury themselves in winter-time in the ground or 
beneath dead vegetation, and in hot climates they assume a torpid condition in the 
hottest and driest season of the year, closing up the aperture of the shells with a 
temporary lid or door (called an epiphragm). Some of these summer-sleepers 
display great tenacity of life, many cases being on record of species which have 
lived for two, three, or even five years in a torpid state, without food of any 
description. Perhaps one of the most interesting instances, is that which has so 
often been quoted, of the specimen of the Egyptian desert-snail (Helix desertorum), 
which was fixed to a tablet in the British Museum for four years, and was dis¬ 
covered to be still living. Some fresh-water forms also are capable of living out of 
their native element for a considerable time. A species of Australian Unionidce 
has been known to exist out of water for over a year, and some kinds of Ampul- 
laria have lived for months after being taken out of their native rivers. 
Reproductive The sexes are distinct in some molluscs, and united in the same 
System. individual in others, but reproduction is in all cases effected by means 
of eggs. These are usually secreted or attached in some suitable position, but in 
some instances the ova are hatched within the oviduct of the parent, as in the 
fresh-water pond-snail ( Vivipara ); and probably in most bivalves the eggs are 
