But we may imagine a plant not fixed to the soil; a plant so constituted as to be capable of moving from place to place; such a plant would not be always in contact with its food, and therefore, as it exercised its faculty of locomotion, it could not but interrupt or suspend its function of nutrition. In a being capable of carrying on these two functions simultaneously, the entire apparatus of the function of nutrition must then be modified. Instead of having spongeolæ fixed immovably in the earth, and spread out in a soil adapted to transmit to these organs nutrient matter in a state fitted for absorption, it must be provided with a reservoir for containing its food, in order that it may carry its aliment about with it in all its changes of place. And such is the modification uniformly found in all animals: an internal reservoir for containing its food is provided, perhaps, for every animal without exception. Even the simplest and minutest creatures with which the microscope has made us acquainted, the lowest tribes of the Infusoria (fig. VIII.), the sentient, self-moving cellules, placed at the very bottom of the animal scale, possess this modification of structure. For a long time it was conceived that these minute and simple creatures were without distinction of parts, that they had no separate organs for the reception and digestion of their food; that they absorbed their aliment through the porous tissue of which their body is composed; that thus, instead of having a separate stomach, their entire body is a stomach, and instead of having even as much as a separate organ for absorption, like the more perfect plant, the whole body might be considered as a single spongeole.