Page:Descent of Man 1875.djvu/282

266 were best able to hold her, have left the greatest number of progeny to inherit their respective advantages.

In some of the lower crustaceans, the right anterior antenna of the male differs greatly in structure from the left, the latter resembling in its simple tapering joints the antennæ of the female. In the male the modified antenna is either swollen in the middle or angularly bent, or converted (fig. 4) into an elegant, and sometimes wonderfully complex, prehensile organ. It serves, as I hear from Sir J. Lubbock, to hold the female, and for this same purpose one of the two posterior legs (b) on the same side of the body is converted into a forceps. In another family the inferior or posterior antennæ are "curiously zigzagged" in the males alone.

In the higher crustaceans the anterior legs are developed into chelæ or pincers; and these are generally larger in the male than in the female,—so much so that the market value of the male edible crab (Cancer pagurus), according to Mr. C. Spence Bate, is five times as great as that of the female. In many species the chelæ are of unequal size on the opposite side of the body, the right-hand one being, as I am informed by Mr. Bate, generally, though not invariably, the largest. This inequality is also often much greater in the male than in the female. The two chelæ of the male often differ in structure (see figs. 5, 6, and 7), the smaller one resembling that of the female. What advantage is gained by their inequality in size on the opposite sides of the