Page:Popular Science Monthly Volume 76.djvu/323

Rh conditions are never anything like three to five fold in a linear dimension.

Other classes of modification experiments on mollusks are not so exact, but are perhaps worth mentioning. Vernon writes that "the permanent effects of temperature on size are probably very considerable among many of the mollusks," and quotes Cooke as stating that "a deficiency of lime, in the composition of the soil of any particular locality, produces very marked effects upon the mollusca which inhabit it; they become small and very thin, occasionally almost transparent." Unfortunately there is no statement that changes can be brought about in one generation, though very likely this is the case. Vernon says, in quoting Costa, page 314:

Regeneration experiments are numerous among the mollusks and indicate, as Przibram points out, that these animals belong, with regard to their relation to the question of regeneration, in the middle rank between the lowest invertebrates and the highest vertebrates. He places mollusks in the fourth of six classes with respect to the power of regeneration, which can regenerate besides the tail, also limbs and organs of sense as long as the connection with the central nervous system is intact.

There have not been many modifications experiments performed upon crustaceans except such as come within the more limited field of regeneration. There is evidence, however, that either different quantities of oxygen or differences in amount of the products of metabolism cause marked variation in the rate of reproduction of Daphnia magna. There is also much evidence that among the Daphnias important changes in the life cycle may be artificially induced. These creatures may be made to continue parthenogenetic reproduction into the winter if kept in a warm place.

According to Irvine and Woodhead crabs can not produce their shells if they are allowed to grow in sea water from which chloride of calcium has been excluded, even if sulphate of lime and chloride of sodium are present. Chloride of calcium is absolutely essential for