and James-Clark and Leidy, both Americans, have made contributions of high quality. Lankester's article in the _Encyclopædia Britannica_ was of epoch-making quality in its philosophical breadth of thought. Delage and Hérouard have given a full account of the Protozoa in their _Traité de Zoologie Concrète_, vol. i. (1896); and A. Lang's monograph in his _Vergleichende Anatomie_, 2nd ed. (1901), contains an admirable analysis of their general structure, habits, and life-cycles, together with full descriptions of well-selected types. Calkins has monographed "The Protozoa" in the Columbia University Biological series (1901). These works of Bütschli, Delage, Lang, and Calkins contain full bibliographies. Doflein has published a most valuable systematic review of the Protozoa parasitic on animals under the title _Die Protozoen als Parasiten und Krankheitserreger_ (1901); and Schaudinn's _Archiv für Protistenkunde_, commenced only four years ago, already forms an indispensable collection of facts and views. The PROTOPLASM of the Protozoa (see p. 5 f.) varies greatly in consistency and in differentiation. Its outer layer may be granular and scarcely altered in Proteomyxa, the true Myxomycetes, Filosa, Heliozoa, Radiolaria, Foraminifera, etc.; it is clear and glassy in the Lobose Rhizopods and the Acrasieae; it is continuous with a firm but delicate superficial pellicle of membranous character in most Flagellates and Infusoria; and this pellicle may again be consolidated and locally thickened in some members of both groups so as to form a coat of mail, even with definite spines and hardened polygonal plates (_Coleps_, Fig. 54, {47}p. 150). Again, it may form transitory or more or less permanent pseudopodia,[61] (1) blunt or tapering and distinct, with a hyaline outer layer, or (2) granular and pointed, radiating and more or less permanent, or (3) branching and fusing where they meet into networks or perforated membranes. Cilia or flagella are motile thread-like processes of active protoplasm which perforate the pellicle; they may be combined into flattened platelets or firm rods, or transformed into coarse bristles or fine motionless sense-hairs. The skeletons of the Protozoa, foreign to the cytoplasm, will be treated of under the several groups. Most of the fresh-water and brackish forms (and some marine ones) possess one or more contractile vacuoles, often in relation to a more or less complex system of spaces or canals in Flagellates and Ciliates.