GENERAL NOTES. 5 
thin soft skin, but in certain groups, such as the large South- 
American Strophochili and the African Achatince, which include the 
largest of known land-molluscs, they are protected by a hardened 
calcareous shell, in some instances fully an inch in diameter. The 
freshwater forms (Limncea and Physa) deposit from thirty to a 
hundred eggs enveloped in a gelatinous mass. 
The number of eggs produced by some Bivalves is enormous. 
The Common Oyster is said to produce a million or more, and the 
American variety ten, or even sixty, times as many. Some of the 
River-Mussels are also very prolific, as many as two millions being 
sometimes the product of a single individual. A small series of 
the eggs of Land-Snails and of the egg-capsules of some marine 
Gastropods is exhibited in side-table case C at the side of the 
Gallery. 
The ova of Mollusca may be gradually developed into the form 
of the parent, or there may be a free-swimming larva, which has 
a circlet of cilia near the anterior pole of its body (so-called 
" Yeliger " larvae), or there may be special larvae, as in the case of 
the Freshwater Mussel, the " Glochidium," as it is called, which 
has a toothed bivalve shell by which it can fix itself to fishes. 
The limits of age of molluscs has been definitely ascertained in Duration 
a few instances only. Most Land-Snails probably live about two ofllfe - 
years, although in confinement some have been kept alive for a 
much longer time. Some of the marine forms live for a considerable 
period, the Common Oyster not attaining full growth until about 
five years old, after which it may continue to live for many years. 
The Giant Clam, a specimen of which is placed in the upright cases 
near the entrance to the Gallery, must, one would think, have a very 
long existence, judging from the size and thickness of the shell. 
All terrestrial molluscs hibernate in cold climates, hiding themselves Hibema- 
away in the ground between roots and similar sheltered places. * ion .^ d 
In tropical countries some assume a state of torpidity (aestivate) 
during the hottest and driest season of the year, closing up the 
aperture of their shells with a temporary lid or door (epiphragm), 
in order to resist the dryness of the atmosphere. Some of these 
" summer-sleepers " are endowed with a remarkable tenacity of life. 
An Australian Pond-Mussel has been known to live a year after 
being removed from the water ; several Land-Snails have revived 
after a captivity of from two to five years, without any food what- 
ever. One of the most remarkable instances of this kind occurred 
in the British Museum. A specimen of Helix desertorum, a common 
