An interesting effect of a sudden fall of temperature was noticed by Semper in connection with the above experiments. Vessels of unequal size, containing specimens of the _Limnaea_, happened to stand before a window at a time when the temperature suddenly fell to about 55° F. The sun, which shone through the window, warmed the water in the smaller vessels, but had no effect upon the temperature of the larger. The result was, that the _Limnaea_ in 2000 cubic cm., which ought to have been 10 mm. long when 25 days old, were scarcely longer, at the end of that period, than those which had lived in the smaller vessels, but whose water had been sufficiently warm. [Illustration: FIG. 37.--Four equally old shells of _Limnaea stagnalis_, hatched from the same mass of ova, but reared in different volumes of water: =A= in 100, =B= in 250, =C= in 600, and =D= in 2000 cubic centimetres. (After K. Semper.)] CHAPTER IV USES OF SHELLS FOR MONEY, ORNAMENT, AND FOOD--CULTIVATION OF THE OYSTER, MUSSEL, AND SNAIL--SNAILS AS MEDICINE--PRICES GIVEN FOR SHELLS The employment of shells as a medium of exchange was exceedingly common amongst uncivilised tribes in all parts of the world, and has by no means yet become obsolete. One of the commonest species thus employed is the ‘money cowry’ (_Cypraea moneta_, L.), which stands almost alone in being used entire, while nearly all the other forms of shell money are made out of portions of shells, thus requiring a certain amount of labour in the process of formation. One of the earliest mentions of the cowry as money occurs in an ancient Hindoo treatise on mathematics, written in the seventh century A.D. A question is propounded thus: ‘the ¼ of 1/16 of ⅕ of ¾ of ⅔ of ½ a dramma was given to a beggar by one from whom he asked an alms; tell me how many cowry shells the miser gave.’ In British India about 4000 are said to have passed for a shilling, but the value appears to differ according to their condition, poor specimens being comparatively worthless. According to Reeve[204] a gentleman residing at Cuttack is said to have paid for the erection of his bungalow entirely in cowries. The building cost him 4000 Rs. sicca (about £400), and as 64 cowries = 1 pice, and 64 pice = 1 rupee sicca, he paid over 16,000,000 cowries in all. Cowries are imported to England from India and other places for the purposes of exportation to West Africa, to be exchanged for native products. The trade, however, appears to be greatly on the decrease. At the port of Lagos, in 1870, 50,000 cwts. of cowries were imported.[205]