NEW STRUCTURES. An interesting example of the origin of a new structure due to change in habits and to resulting strains and movements is the formation of bivalve shells in animals of classes so unlike as the bivalve mollusks (Pele cypoda) and the ostracod, phyllopod, and phyb loearidan crustaceans. The shell or carapace has become folded into two valves to protect the body. The valves are opened and closed by the relaxation or contraction of one or two peculiar muscles, the adductors. These muscles are not with any Inuseles in other classes, and at least in bivalve mollusks they are prob ably developed from the mantle muscle as a eon. sequence of the conditions of the ease. The sev eral types thus occurring in different brandies or phyla "is a strong proof that common forces acting on all alike have induced the resulting, form" (Jackson). There are also good examples of mimiery or 'convergence,' and many so-called eases of mimicry are undonbt edly merely ex amples of such convergence or similarity of form due to the subjection of animals of quite different groups to identical habits or conditions.
Now, as the order of Crustacea is founded in pat on the nature of the carapace, or of the limbs, whether adapted for walking, swimming, or biting, etc., the ordinal characters are evi dently due to the different uses to which these parts are adapted. It is so with the (gasses of mollusks; the bivalves are secondary forms which by change of habits gradually evolved from some wormlike ancestor. The gastropods, with their unsymmetrical shells, and the creeping or swim ming cephalopods, with their closely coiled shells ( when a shell is present), are clearly the result of the use of certain parts, the disuse of others. SO it is with the orders of mammals and birds, and the form of man is mainly due to the disuse of his feet in climbing, to his erect position, and to the use and exercise of his brain.