The body consists of a soft granular protoplasmic substance, the middle of which is occupied by a large oval mass called the “nucleus;” while, at its hinder end, is a “contractile vacuole,” conspicuous by its regular rhythmic appearances and disappearances. Obviously, although the _Colpoda_ is not a monad, it differs from one only in subordinate details. Moreover, under certain conditions, it becomes quiescent, incloses itself in a delicate case or _cyst_, and then divides into two, four, or more portions, which are eventually set free and swim about as active _Colpodæ_. But this creature is an unmistakable animal, and full-sized _Colpodæ_ may be fed as easily as one feeds chickens. It is only needful to diffuse very finely ground carmine through the water in which they live, and, in a very short time, the bodies of the _Colpodæ_ are stuffed with the deeply-coloured granules of the pigment. And if this were not sufficient evidence of the animality of _Colpoda_, there comes the fact that it is even more similar to another well-known animalcule, _Paramæcium_, than it is to a monad. But _Paramæcium_ is so huge a creature compared with those hitherto discussed—it reaches 1/120 of an inch or more in length—that there is no difficulty in making out its organisation in detail; and in proving that it is not only an animal, but that it is an animal which possesses a somewhat complicated organisation. For example, the surface layer of its body is different in structure from the deeper parts. There are two contractile vacuoles, from each of which radiates a system of vessel-like canals; and not only is there a conical depression continuous with a tube, which serve as mouth and gullet, but the food ingested takes a definite course, and refuse is rejected from a definite region. Nothing is easier than to feed these animals, and to watch the particles of indigo or carmine accumulate at the lower end of the gullet. From this they gradually project, surrounded by a ball of water, which at length passes with a jerk, oddly simulating a gulp, into the pulpy central substance of the body, there to circulate up one side and down the other, until its contents are digested and assimilated. Nevertheless, this complex animal multiplies by division, as the monad does, and, like the monad, undergoes conjugation. It stands in the same relation to _Heteromita_ on the animal side, as _Coleochæte_ does on the plant side. Start from either, and such an insensible series of gradations leads to the monad that it is impossible to say at any stage of the progress—here the line between the animal and the plant must be drawn.