Page:Popular Science Monthly Volume 35.djvu/268

 divided, and each of the newly formed organisms receives its share. These little beings are particularly entitled to our attention, because each higher organism is also originally evolved from a naked egg-cell, and devoid of any membrane or cuticle. Moreover, such a cell shows essentially the same simple structure, and moves about in the same manner as the amœba—that is, by the aid of pseudopodia.

Hence the amœbæ are one of the lowest forms of organisms known; they have remained on the lowest plane of development, and, if one accepts, with Lamarck and Darwin, the evolution of the animal world from a simplest beginning, these creatures must be regarded as the original progenitors of all forms of animal life. Of course, every one is at liberty to doubt such progressive evolution of organized beings; but this much is certain, the individual development of each proceeds from a primary state, which is not greatly different from the structure of these amœbæ.

Fig. 2 represents a shell-bearing amoeba (Euglypha), which is also to be found in great numbers in the mud of wells. In this organism the naked sarcode, which consists of a substance similar to albumen, is covered and protected by a membranous envelope, or "carapace," from which, through an opening, the pseudopodia (ps) appear. The little organism pictured in Fig. 3 stands in close relationship to the preceding (Centropyxis aculeata.) Its shell, made up of diatoms and fragments of small particles of stone, shows thorn-like protuberances.



Fig. 4 makes us acquainted with the appearance of the flagellate infusoria. These are animals that have a delicate body, which can be contracted, and at one end of which there is a filament (gf) which is constantly in motion, and with which certain movements are executed. This filament is in reality nothing else than a long