The foregoing illustrations are taken from a few of our commonest and most easily obtainable _Crustacea_; but they amply suffice to exemplify the manner in which the conception of a plan of organization, common to a multitude of animals of extremely diverse outward forms and habits, is forced upon us by mere comparative anatomy. Nothing would be easier, were the occasion fitting, than to extend this method of comparison to the whole of the several thousand species of crab-like, crayfish-like, or {279} prawn-like animals, which, from the fact that they all have their eyes set upon movable stalks, are termed the _Podophthalmia_, or stalk-eyed _Crustacea_; and by arguments of similar force to prove that they are all modifications of the same common plan. Not only so, but the sand-hoppers of the sea-shore, the wood-lice of the land, and the water-fleas or the monoculi of the ponds, nay, even such remote forms as the barnacles which adhere to floating wood, and the acorn shells which crowd every inch of rock on many of our coasts, reveal the same fundamental organization. Further than this, the spiders and the scorpions, the millipedes and the centipedes, and the multitudinous legions of the insect world, show us, amid infinite diversity of detail, nothing which is new in principle to any one who has mastered the morphology of the crayfish. Given a body divided into somites, each with a pair of appendages; and given the power to modify those somites and their appendages in strict accordance with the principles by which the common plan of the _Podophthalmia_ is modified in the actually existing members of that order; and the whole of the _Arthropoda_, which probably make up two-thirds of the animal world, might readily be educed from one primitive form. And this conclusion is not merely speculative. As a matter of observation, though the _Arthropoda_ are not all evolved from one primitive form, in one sense of the words, yet they are in another. For each can be traced {280} back in the course of its development to an ovum, and that ovum gives rise to a blastoderm, from which the parts of the embryo arise in a manner essentially similar to that in which the young crayfish is developed.