These creatures fix themselves to their movable supports by means similar to those by which other creatures secure themselves to stationary supports. Thus, some of these do so by means of expanded disks, which fit accurately to the supporting surface, while certain parasites fix themselves by means of ingrowing prolongations or root-like processes, as in the _Rhizocephala_.[72] Others, again, adhere by the intervention of hooks and suckers, and this is especially the case with such as fix themselves internally and live perpetually bathed (as the tape-worms[73] do) in the nutritious fluids contained within the bowels of the creatures they infest. Terrestrial mobile organisms can, of course, only be moved by their own efforts, or by the efforts of other organisms. The simplest terrestrial locomotion is like that of the aquatic _Amœba[74] primitiva_, and is performed by land _Amœbæ_; and the curious plant _Myxomycetes_[75] also moves in a substantially similar manner. This very curious organism consists of a net-work of protoplasmic threads, which spread over decaying leaves and stems. The threads exhibit streams of granules flowing within them, and they give out processes like pseudopodia, while the whole complex mass can slowly creep over a supporting surface, which it thus slowly flows over by its branching processes. Other lowly plants propel themselves by means of a pair of filamentary protoplasmic threads, which vibrate actively, and are therefore called vibratile cilia. As an example may be mentioned the _Protococcus[76] nivalis_, the little spheroidal alga, which abounds on Alpine summits and in Arctic regions. As in aquatic, so in terrestrial organisms, external form is intimately related to modes of motion. Thus, locomotion may be effected by undulations of the whole body, as often in serpents and terrestrial vermiform animals. It may, on the contrary, be effected by the action of levers projecting from the surface of the body, _i.e._, by limbs, and these may be multitudinous and minute, as in hundred legs and thousand legs, or few and large, as in beasts. Moreover, the motions may be movements of pulling or of pushing, or by combinations of these, or by jumps, which may be effected in various manners, the consideration of which will find a fitting place in an Essay devoted to "Motion."