GENERAL CHARACTERISTICS 3291 



of the mollusk. 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 colors, and 

 its test merely of some uniform sombre hue. Very gorgeously painted shells are, 

 however, generally indicative of highly colored inhabitants. 



That light and warmth are great factors in the production of brilliant coloring 

 is beyond question. This is conclusively proved by the results of deep-sea dredg- 

 ing. Whenever great depths are reached, where darkness reigns, and the water is 

 intensely cold, the molluskan inhabitants are without color, 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 mollusks is 

 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 three hundred 

 or four hundred pounds 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 mollusks 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 mollusks, 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 be- 

 neath 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 dis- 

 play 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 discovered 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 Unionidee has been known 

 to exist out of water for over a year, and some kinds of Ampullaria have lived for 

 months after being taken out of their native rivers. 



The sexes are distinct in some mollusks, and united in the same in- 

 dividual in others, but reproduction is in all cases effected by means of 

 eggs. These are usually secreted or attached in some suitable posi- 

 tion, 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 



