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. Moving subterranean organisms, inasmuch as they must penetrate through a dense and highly-resisting substance, must evidently either have forms which offer little resistance--reducing friction to a minimum--or must be provided with special means of penetrating such substance. Evidently the least resisting form is presented by a body much elongated, rounded, and more or less attenuated at the advancing end, which end has to effect the requisite penetration. This is the form of the earth-worm--a form which is approximated to by a variety of creatures which have not the least affinity of nature with it, but only more or less resemble it as regards its dwelling-place and mode of locomotion. Such, for example, are the curious serpents called _Typhlops_,[79] and such are the legless lizards[80] (_Anguis_), and such, again, are the simpler vermiform animals allied to frogs, called _Cæciliæ_.[81]