Did space permit, it would be possible to point out among the _Protozoa_, various cases analogous to that of the _Arcella_; which may be described as like a microscopic Limpet, having a sarcode body of which the upper surface has become horny, while the lower surface with its protruding pseudopodia, retains the primitive jelly-like character. That differentiations of this kind have been gradually established among these minute creatures through the unlike relations of their parts to the environment, is an inference supported by a form which, while the rest of the body has a scarcely distinguishable coating, “agrees with _Arcella_ and _Difflugia_ in having the pseudopodia protrusible from one extremity only of the body.” Many parallel specializations of surface among aggregates of the second order might be instanced from the _Cœlenterata_. In the _Hydra_, the ectoderm presents over its whole area no conspicuous unlikenesses; but there usually exist in the hydroid polypes of superior types, decided contrasts between the higher and lower parts. While the higher parts retain their original characters, the lower parts excrete hard outer layers yielding support and protection. Various stages of the differentiation might be followed. “In _Hydractinia_,” says Prof. Green, this horny layer “becomes elevated at intervals to form numerous rough processes or spines, while over the general surface of the ectoderm its presence is almost imperceptible.” In other types, as in _Cordylophora_, it spreads part way up the animal’s sides, ending indefinitely. In _Bimeria_ it “extends itself so as to enclose the entire body of each polypite, leaving bare only the mouth and tips of the tentacles.” While in _Campanularia_ it has become a partially-detached outer cell, into which the creature can retract its exposed parts. But it is as needless as it would be wearisome to trace through the several sub-kingdoms the rise of these multiform contrasts, with the view of seeking interpretations of them. It will suffice if we take a few groups of the illustrations furnished by the higher animals. § 292. We may begin with those modifications of surface which subserve respiration. Though we ordinarily think of respiration as the quite special function of a quite special organ, yet originally it is not so. Little-developed animals part with their carbonic acid and absorb oxygen, through the general surface of the body. Even in the lower types of the higher classes, the general surface of the body aids largely in aërating the blood; and the parts which discharge the greater part of this function are substantially nothing more than slightly altered and extended portions of the skin.