There are a multitude of aquatic creatures which cannot be properly spoken of as either “fixed” or “mobile,” for they are in fact both. They are creatures which move about by the help of others, being themselves fixed to other creatures which are actively locomotive.
Thus, sea-snails, lobsters, fishes, whales, and even ships, bear about with them sometimes lowly-organized plants; but often other animals, permanently fixed to and growing parasitically upon them and having the shape of their body suited to their peculiar situation.
Often such parasites form flattened encrustations on their involuntary hosts—as is the case with the acorn shells or sessile barnacles.[70] Others have elongated bodies, which stream through the water with the motions of the creatures carrying them. We see this in confervoid growths, also in ordinary barnacles, and in certain modified crab-like creatures, such as Lerneocera.[71]
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.”
Again, terrestrial, like aquatic, organisms often involuntarily carry about with them other living creatures which have fixed themselves to their bodies. Thus, the fruits, or seeds, of many plants (as, e.g., those of the common Agrimony, Agrimonia eupatoria) are beset with hooks or bristles which readily adhere to the coats of passing animals, and so gain a greater diffusion than they could otherwise obtain. A very remarkable form of the kind is Martynia proboscidea (called Testa di Quaglia by the Italians), which has a pair of curved and pointed processes like the tusks of an elephant, which are several inches long. It is notorious for adhering to clothes, &c. Other noteworthy plants are Uncaria procumbeus, or the grapple plant of South Africa and Harpagophytum,[77] the fruit of which is provided with hooked processes. Those of Harpagophytum spread out in all directions, and are of different lengths, with sharp hooks, variously turned, so that its power of clinging is extreme. The seed, with all its processes, is so large as to fill the hand when grasped. It is said to cause the death of the lion. Having adhered to that beast’s skin, the irritation produced and the impossibility of getting it off at last induces the lion to bite it, and once in his mouth he cannot remove it, and so the animal dies miserably.
Some animals fix themselves much as these seeds of plants do. Amongst them are the parasites known as tics which fix themselves with great tenacity by the appendages of their mouths. Other parasites—like the itch insect[78] and forms allied to it—have hooked processes and stiff, hard bristles, which are at once very irritating and very adherent. Creatures are also carried about inside others, as is the case with the seeds of many plants. These are disseminated by birds which have swallowed but have not digested such seeds, and in an analogous manner the great tape-worm group becomes also widely diffused.