Among the higher metaphyta we find parasitism in many different families, especially orchids, rhinanthacea (_orobranche_, _lathraca_), convolvulacea (_cuscuta_), aristolochiacea, loranthacea (_viscum_, _loranthus_), rafflesiacea, etc. These various kinds of flowering-plants often grow to resemble each other by convergence (that is to say, by their common adaptation to parasitic life); they lose their green leaves, the plasmodomous chlorophyll of which is of no further use to them. Frequently rudimentary leaves are left on them in the form of colorless scales. For the purpose of clinging to the plants they live on, and penetrating into their tissues, they evolve special clinging apparatus (haustoria, suctorial cups, creepers). Their stalks and roots are also modified in a characteristic way. The whole productive force of these parasites is expended on their sexual organs; _rafflesia_ has the largest flowers there are, more than a yard in diameter. Parasitism in the metazoa (in all groups) is even more frequent and interesting than in the metaphyta. The mollusks and echinoderms show the least disposition for it, and the platodes, vermalia, and articulates the most. Even among the gastræada, the common ancestral group of the metaphyta, we find parasites (kyemaria and gastremaria). The protection they find inside their hosts is probably the reason why these oldest of the metazoa have remained unchanged to the present day. Real parasites are not numerous among the sponges and cnidaria. But they are very numerous among the platodes. The suctorial worms (_trematodes_) live partly externally (as ectoparasites) on other animals and partly inside them (as endoparasites), and produce serious diseases in them. They have lost the vibratory coat of their free-living ancestors, the turbellaria, and acquired clinging apparatus instead. The tape-worms (_cestodes_), which live entirely in the interior of other animals, and are descended from the suctorial worms, have lost their gastro-canal; they are nourished by imbibition through the skin. The same degeneration is found in the itchworms (_acanthocephala_) among the vermalia, the parasitic snails (_entoconcha_) among the mollusks, and the root-crabs (_rhizocephala_) among the crustacea.