MOLLUSCA. 
493 
to deeper water, and in the following spring return to the tidal rocks, attain their full growth 
early in the summer, 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 spiral 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 enlarging 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, tridacna^ 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 mollusca 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 effectually 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 much less prolific. The land-snails have fewer enemies, and, for- 
tunately, lay fewer eggs. 
Finally, the Mollusca 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 Avhich 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 are clustered in groups, with spaces pervad- 
ing the interior for the free passage of sea-water ; and the 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, designed, like that of the opossum, to protect 
them until they are strong enough to shift for themselves. 
In the Introduction to this work (Vok L, 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 proceed to describe some of the more interesting species of this great division of 
Animated Nature, arranging them, according to the Classification in Vol. I., pp. 27, 28, in seven 
classes : Cephalopoda, Gasteropoda, Pteropoda, Palliobranchiata, Lamellibranchiata, Tanicata, 
and Bryozoa. 
* See Introduction to " Rudimentary Treatise on Recent and Fossil Shells, by S. P. Woodward," London, 1851, from 
which we have chiefly derived this general view of the Mollusca. 
