often continue adherent for a long time, forming strings or other temporary aggregations of such organisms. In _Protococcus_ we meet with one of the lowest order. Its colour is green, which, as in all other higher plants also, is due to the presence in its protoplasm of a colouring matter called _chlorophyll_, either diffused or aggregated in certain denser granules of protoplasmic substance. Protococcus may be smaller or much larger than the yeast plant, it is spheroidal, and its protoplasm is enclosed in a tough case of cellulose, which, however, it may not nearly fill, while the long cilia may protrude through it and propel the whole organism by their reiterated lashings. It has been already said that a vegetable may temporarily exist as a particle of bioplasm without any cell-wall, and such is the case with _Protococcus_, the cellular envelope of which occasionally disappears. More remarkable still is the form already referred to under the name _Myxomycetes_,[88] which, for part of its existence, is the form of an indefinitely-shaped, naked protoplasmic mass.[89] Living creatures which consist of a single cell may present, nevertheless, a considerable complication of structure. Thus, an organism as simple as the _amœba primitiva_, before noticed, may have the power of forming, or, as it is technically called, _secreting_, from its own substance and its surrounding medium a most complex supporting skeleton of calcareous or silicious nature. It may have its outer envelope so markedly differentiated from its inner as to require a distinct designation as _exosarc_, while it may give rise in its interior not only to a nucleus and nucleolus, but to two regularly formed cavities with the power of rythmical pulsation, and one definite portion of its external wall may be perforated to form a permanent mouth instead of as in such forms as _Amœba_, any part serving indifferently as a mouth and every portion having similar functions without differentiation. All these and other complications of structure may arise by direct growth and transubstantiation of the single cell into the various physically and chemically different parts. Again, a living creature which is fixed may so extend itself as to simulate stem, roots, and branches, and yet remain essentially simple, consisting merely of one greatly enlarged and complicated cell. Thus, a unicellular plant may take on a great complexity of form while still remaining purely unicellular. It may assume the form of a stem with roots and leaves. An example of such we may see in the genus _Caulerpa_,[90] which, although unicellular, simulates in its outline the fern called _Blechnum_.