Descending the scale to the very bottom, we reach a class of animals, the Protozoa, which cannot be separated in many cases from the Protophyta by these distinctions, since many of the former have no digestive cavity, nor the slightest trace of a nervous system, while many of the latter possess the power of active locomotion. As to external configuration, no certain rules can be laid down for separating animals and plants, many of the lower plants, either in their earlier stages, or in their maturity, being exactly similar in form to some of the lower animals. This is the case with some of the Algæ, which resemble very closely in form certain Infusorian animalcules. Again, many undoubted animals, which are rooted to solid objects in their adult state, are so plant-like in appearance as to be popularly regarded as vegetables. The Sea-firs, and the more highly organized Flustras or Sea-mats, which are usually considered as sea-weeds by sea-side visitors, are a few of many examples that might be taken from the so-called Hydroid Zoöphytes. No decided distinction between animals and plants can be drawn as to their minute internal structure, both alike consisting of molecules, of cells, or of fibres. Some decided, though not universal, differences exist in chemical composition. Plants exhibit a decided predominance of ternary compounds, or compounds which, like sugar, starch and cellulose, are made up of the three elements, carbon, hydrogen and oxygen, but are, comparatively speaking, poorly supplied with quaternary compounds, or those which contain an additional element of nitrogen. Animals, on the contrary, are rich in quaternary nitrogenized compounds, such as albumen or fibrin. Still, in both kingdoms we find nitrogenized and non-nitrogenized compounds, and it is only in the proportion which these sustain to each other in the organism that animals differ in any way from plants. Before the invention of the microscope, no independent voluntary movements, if we except the opening and closure of flowers, and their turning towards the sun, the drooping of the leaves of sensitive plants under irritation, and some other kindred phenomena, were known in plants. Now, however, we know of many plants which are endowed, either when young or throughout life, with the power of effecting voluntary movements apparently as spontaneous and independent as those performed by the lower animals, the movements being brought about by means of little vibrating cilia, or hairs, with which a part or the whole of the surface is furnished. When it is added that many animals are permanently rooted, in their fully-grown condition, to solid objects, it will at once be apparent that no absolute distinction can be made between animals and plants merely because of the presence or absence of independent locomotive power.