GENERAL CHAR A CTERS. 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 



