MOLLUSC A. 493 



to deeper water, and in the following spring return to the tidal rocks, attain their full growth 

 earlv in the sunnner, and after spawning-time disappear. 



The land-snails are mostly biennial; hatched in the summer and autumn, they are half-grown 

 by the winter-time, and acquire their full growth in the following spring or summer. In con- 

 finement, a garden-snail will live for six or eight years; but in their natural state it is probable 

 that a great many die in their second winter, for clusters of empty shells may be found, adhering 

 to one another, under ivied walls, and in other sheltered situations ; the animals having perished 

 in their hybernation. Some of the^piral sea-shells live a great many years, and tell their age in 

 a very plain and interesting manner, by the number of fringes on their whirls ; the contour of 

 the ranella and murex depends on the regular recurrence of these ornaments, which occur after 

 the same intervals in well-fed individuals, as in their less fortunate kindred. The ammonites 

 appear, by their whirls or periodic mouths, to have lived and continued growing for many 

 years. 



Many of the bivalves, like the mussel and cockle, attain their full growth in a year. The 

 oyster continues enlai'ging his shell by annual shoots, for four or five years, and then ceases to 

 grow outward ; but very aged specimens may be found, especially in a fossil state, with shells 

 an inch or two in thickness. The giant-clam, tridocna, which attains so large a size that poets 

 and sculptors have made it the cradle of the sea-goddess, must enjoy an unusual longevity ; 

 living in the sheltered lagoons of coral islands, and not discursive in its habits, the corals grow 

 up around, until it is often nearly buried by them ; but although there seems to be no certain 

 limit to its life, though it may live a century for all that we know, yet the time will probably 

 come when it will be overgrown by its neighbors, or choked with sediment. 



The fresh-water inollusca of cold climates bury themselves during winter in the mud of their 

 ponds and rivers ; and the land-snails hide themselves in the ground, or beneath moss and dead 

 leaves. In warm climates they become torpid during the hottest and driest part of the year.* 



The permanency of the shell-bearing races is effectuallv provided for by their extreme fecund- 

 ity ; and though exposed to a hundred dangers in their early life, enough survive to repeople 

 the land and sea abundantly. The spawn of a single doris may contain six hundred thousand 

 eggs; a river mussel has been estimated to produce three hundred thousand young in one sea- 

 son, and the oyster cannot be nnich less prolific. The land-snails have fewer enemies, and, for- 

 tunately, lay fewer eggs. 



Finally, the MoUusca exhibit the same instinctive care with insects and the higher animals in 

 placing their eggs in situations where they will be safe from injury, or open to the influences of 

 air and heat, or surrounded by the food which the young will require. The tropical bulimi 

 cement leaves together to protect and conceal their large, bird-like eggs; the slugs bury theirs 

 in the ground; the oceanic-snail attaches them to a floating raft; and the argonaut carries them 

 in her frail boat. The horny capsules of the whelk arc clustered in groups, with spaces pervad- 

 ing the interior for the free passage of sea-water; and tlie nidamental ribbon of the doris and 

 eolis is attached to a rock, or some solid surface from which it will not be detached by the 

 waves. The river-mussel and cyclas carry their parental care still further, and nurse their young 

 in their own mantle, or in a special marsupium, desio-ned, like that of the opossum, to protect 

 them until they are strong enough to shift for themselves. 



In the Introduction to this work (Vol. I., pp. 21, 22) we have given a brief view of the 

 structure and physiology of the Mollusca; inviting the reader's attention to the facts there 

 given, we now procee<l to describe some of the more interesting species of this great division of 

 A^nimated Nature, arranging them, according to the Cl.-issification in Vol. I., pp. 27, 28, in se\eu 

 classes: Cqyhalopoda, Gasteropoda, Plerojjoda, Palliobruuckiuta, LumcUihranchlata, Tuniaita, 

 and, Bryozoa. 



•••■ S'^e Introflnction ir> " RaiUmenUry Treatise on Recent and Fonsll Shells, by S. P. Woodward," London, IS.Jl, from 

 v/hich we have chiefly derived this general view of the Mollusca. 



