The unicellular protophyta exhibit the same form of metabolism and plasmodomism as the familiar green cells of the tissue-plants; but in most of the protozoa we find special features of nutrition and plasmophagy. The great class of the rhizopods is distinguished by the fact that their naked plasma body can take in ready-formed solid food at any point of its surface. On the other hand, most of the infusoria have a definite mouth-opening in the outer wall of their unicellular body, and sometimes a gullet-tube as well. Besides this cell-mouth (_cytostoma_) we usually find also a second opening for the discharge of indigestible matter, a cell-anus (_cytopyge_). Metabolism in the tissue plants (metaphyta) forms a long gradation from very simple to very complicated arrangements. The lowest and oldest thallophyta, especially the simplest algæ, are not far removed from the communities of protophyta, and, like these, are merely definitely grouped colonies of cells. The social cells which form their most rudimentary tissue are quite homogeneous, with no differentiation beyond that of sex. The thallus or bed-formation consists in the simplest specimens of plain or branched fine threads, consisting of rows or chains of homogeneous cells (so _conferva_ among the green, _ectocarpus_ among the brown, and _callithamnion_ among the red algæ). Other algæ (such as the ulva) form thin leaf-shaped forms of the thallus, a number of homogeneous cells lying side by side along a level. In the larger algæ compact tissue-bodies are formed, in which frequently firmer rows of cells exhibit the rudiments of fibres; and the thallus divides, as in the cormophyta, into root, stalk, and leaves. There is also a trophic differentiation, the fibres undertaking special functions of nutrition (the conduction of the sap). The same must be said of the mosses (_bryophyta_). Their lowest forms (_ricciadinæ_) are close akin to the algæ; the highest mosses (the _mnium_ and _polytrichum_, for instance) approach the cormophyta. Many botanists comprise these lower plants--algæ, fungi, and mosses--under the title of "cell-plants" (_cytophyta_), and oppose the higher plants--ferns and flowering-plants--to them as "vascular plants" (_angiophyta_), because they have complex fibres or sap vessels. This distinction has a phylogenetic significance similar to the division between cœlenteria and cœlomaria in the animal kingdom.