GENERAL NOTES. 



largest of known land-molluscs, they are protected by a hardened 

 calcareous shell, in some instances fully an inch in diameter. The 

 freshwater forms {Limn(ea 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 

 Eiver-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 G 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 

 " Veliger" 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 ™l"®- 

 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 Hiberna- 

 away in the ground between roots and in similar sheltered places. 

 In tropical countries some assume a state of torpidity (sestivate) 

 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, the common 

 Desert-Snail of Egypt, was fixed to a tablet in March, 1846, and in 

 the same month of the year 1850 it was discovered to be alive. 



tion and 

 torpidity. 



