Page:Popular Science Monthly Volume 8.djvu/675

Rh to Heteromita, and, like it, grouped under the general name of "Monads," which, nevertheless, can be observed to take in solid nutriment, and which therefore have a virtual, if not an actual, mouth and digestive cavity, and thus come under Cuvier's definition of an animal. Numerous forms of such animals have been described by Ehrenberg, Dujardin, H. James Clark, and other writers on the Infusoria.

Indeed, in another infusion of hay in which my Heteromita lens occurred, there were innumerable infusorial animalcules belonging to the well-known species Colpoda cucullus.

Full-sized specimens of this animalcule attain a length of between $1⁄300$ or $1⁄400$ of an inch, so that it may have ten times the length and a thousand times the mass of a Heteromita. In shape it is not altogether unlike Heteromita. The small end, however, is not produced into one long cilium, but the general surface of the body is covered with small, actively-vibrating ciliary organs, which are only longest at the small end. At the point which answers to that from which the two cilia arise in Heteromita, there is a conical depression, the mouth; and in young specimens a tapering filament, which reminds one of the posterior cilium of Heteromita, projects from this region.

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 Colpodœ.

But this creature is an unmistakable animal, and full-sized Colpodœ 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 Colpodœ are stuffed with the deeply-colored 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 organization in detail; and in proving that it is not only an animal, but that it is an animal which possesses a somewhat complicated organization. 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