V aria lion. 17 



shell has been shown in molluscs inhabiting lakes and lagoons. 

 Changes in temperature may also induce various changes, e.g. in 

 colour and in the shape of the shell. Thus the Slug Amalia gracilis 

 at a constant temperature of 36° C. is yellowish brown, while 

 il becomes black if the temperature is decreased to 10° C. 



Other factors which we can distinguish with less certainty 

 sometimes produce remarkable changes in structure. The shell 

 of a form of Limnaea andersoniana found in ponds in the Shan 

 States has a short spire, the body whorl large and swollen, and 

 the aperture wide. Another form of the same species found in 

 streams has a narrower shell, with the spire longer and the 

 aperture smaller. Similar differences exist between two forms of 

 European Limnaea {L. ovata and L. peregra), and it is stated that 

 it is possible to change the shell-form of the one into that of the 

 other by transferring the eggs of L. ovata to running water or 

 those of L. peregra to still water. 



While such variation appears to be the direct result of changes 

 in environment there are others which cannot be ascribed to this 

 cause ; for instance, the diversity of colour- marking seen in 

 certain common Snails (Tachea nemoralis and T. hortensis) 

 (Case No. 135). 



Economic Importance. 



Mollusca have been used as food by man at least since the 

 Palaeolithic period, and in every region of the world. Although 

 the oyster is, and has been since Roman times, the most highly 

 esteemed, several other less valuable species are very largely 

 consumed in this country. For example, in 1922, the following 

 quantities of edible molluscs were delivered at Billingsgate 

 Market, London: oysters, 1,207 tons; mussels, 1,888 tons; 

 periwinkles, 3,245 tons ; whelks, 889 tons ; cockles, 88 tons. 

 (Data kindly supplied by the Fishery Department, Fishmongers' 

 ( (impany.) 



The edible Snail is sufficiently important in France to make its 

 cultivation a regular industry. Mollusca may also be of impor- 

 tance to man as the food of edible fishes ; for example, Lamelli- 

 branchs form 80 per cent, of the food of the Plaice. 



The most valuable commercial products furnished by Mollusca 

 are pearls, which are obtained from the Pearl Oyster (Margariti- 

 fera) and a few other Lamellibranchs. An account of the forma- 

 tion of pearls will be found on p. 39. 



.Mother-of-pearl derived from the shells of several genera of 

 marine and freshwater Lamellibranchs is an important article 

 of commerce for button-making and other purposes. The culti- 

 vation of many species of River Mussels (Unionidae) in the 

 United States for the production of pearl shell is an industry 

 which in recent years has assumed large proportions. 



B 



