§ 102. A general truth of much significance is exhibited in these classifications. On observing the natures of the attributes which are common to the members of any group of the first, second, third, or fourth rank, we see that groups of the widest generality are based on characters of the greatest importance, physiologically considered; and that the characters of the successively-subordinate groups, are characters of successively-subordinate importance. The structural peculiarity in which all members of one sub-kingdom differ from all members of another sub-kingdom, is a peculiarity that affects the vital actions more profoundly than does the structural peculiarity which distinguishes all members of one class from all members of another class. Let us look at a few cases. We saw (§ 56), that the broadest division among the functions is the division into "the _accumulation of energy_ (latent in food); the _expenditure of energy_ (latent in the tissues and certain matters absorbed by them); and the _transfer of energy_ (latent in the prepared nutriment or blood) from the parts which accumulate to the parts which expend." Now in the lowest animals, united under the general name _Protozoa_, there is either no separation of the parts performing these functions or very indistinct separation: in the _Rhizopoda_, all parts are alike accumulators of energy, expenders of energy and transferers of energy; and though in the higher members of the group, the _Infusoria_, there are some specializations corresponding to these functions, yet there are no distinct tissues appropriated to them. Similarly when we pass from simple types to compound types--from _Protozoa_ to _Metazoa_. The animals known as _Coelenterata_ are characterized in common by the possession of a part which accumulates energy more or less marked off from the part which does not accumulate energy, but only expends it; and the _Hydrozoa_ and _Actinozoa_, which are sub-divisions of the _Coelenterata_, are contrasted in this, that in the second these parts are much more differentiated from one another, as well as more complicated. Besides a completer differentiation of the organs respectively devoted to the accumulation of energy and the expenditure of energy, animals next above the _Coelenterata_ possess rude appliances for the transfer of energy: the peri-visceral sac, or closed cavity between the intestine and the walls of the body, serves as a reservoir of absorbed nutriment, from which the surrounding tissues take up the materials they need. And then out of this sac originates a more efficient appliance for the transfer of energy: the more highly-organized animals, belonging to whichever sub-kingdom, all of them possess definitely-constructed channels for distributing the matters containing energy. In all of them, too, the function of expenditure is divided between a directive apparatus and an executive apparatus--a nervous system and a muscular system.