The same thing is shown by the legs and other appendages of insects and crustaceans, which are adapted for the most diverse functions, and the individual sections of which must be correlated if the function is to be possible. Let us consider only the claw structures in crustaceans and scorpions. Here, too, it seems as if the outgrowth of the last joint of the leg, which functions as the arm of the claw, must have arisen as a direct effect of use, through the pressure of an object held fast by the last joint, the movable half of the claw. Frequently, moreover, tooth-like protuberances occur on the fixed blade of the claw (Fig. 103). But how could these have arisen as a direct effect of pressure, since they are always preformed during the soft state of the appendage _before use_, and are only made use of after it is fully hardened. The soft crustaceans, the so-called 'butter-crabs' which have just cast their shells, creep carefully away and avoid using their limbs until they have become hard again. Here, too, we have the co-adaptation of two parts which vary independently, and which cannot be affected by the Lamarckian principle. [Illustration: FIG. 103. Claw (_Sch_) on the leg of a 'Beach-fly,' an Amphipod Crustacean (_Orchestia_). _I_, _II_, the two first joints. _uA_, the lower blade of the claw, a non-mobile prolongation of the penultimate joint. _oA_, the upper blade of the claw, the movable last joint; the tubercles and indentations of the two blades fit one another. After F. Müller.]