Encysted Vorticella, showing the obliteration of special organs by the advancement of the process.—Pritchard.
These changes are exhibited in the annexed cuts, which are copied from known authorities. By careful observation of the bodies of Vorticellids, a contractile vesicle may be observed, which appears to cause a movement of fluids, that is probably connected either with respiration or secretion.
Another piece of apparatus in this family, but not confined to it, is the so-called nucleus, which in this case is of a horseshoe shape and granular texture, and greater solidity than the surrounding parts. The functions of this organ formed the subject of various conjectures, but it is now generally held to be an ovary.
Vorticella microstoma, in process of encystment, 300 linear; in the last the inclosing tunic is plainly developed.—Stein.
In common with many of the lower animals, the Vorticellids have three ways of multiplying their race. One by fission, or division of their bodies: another by buds, somewhat analogous to those of plants; and another by reproductive germs. These processes will come again under our notice, and we shall leave the Vorticellids for the present by observing that if they are fed with a very small quantity of indigo or carmine, the vacuoles or spaces, into which their nutriment passes, will be clearly observed. Ehrenberg thought in these and similar creatures that every vacuole was a distinct stomach, and that all the stomachs were connected by an intestinal canal; hence his name Polygastrica, or many stomached. In these views he has not been followed by later observers, and it is probable he was misled, partly by pushing the process of reasoning from the analogies of higher animals much too far, and partly by the imperfection of the glasses he employed.
Rotifer vulgaris.—A, mouth, or gizzard; B, contractile vesicle.—Micrographic Dictionary.
N.B.—When the cilia and tail part are retracted, and the body shortened, the creature assumes an obtuse oval form.
Having thus briefly considered the Vorticellids we must turn to the wheel-bearer, who belongs to a higher race than even the ciliated Protozoa. We left her crawling about with her snout or proboscis protruded, but now she has moored herself by her tail-foot, pulled in her nose, and put out two groups of cilia, which look like revolving wheels, and a little below them is seen a gizzard in a state of active work. After a little while she swims away with her wheels going, and her tail, forked at the end, is found to be telescopic, or capable of being pulled in and out. As the cilia play, the neighbouring water is agitated, and the multitudes of small objects are brought by the whirlpools within her ravenous maw. But the strangest thing of all is that inside her body is seen a young one; in this case a large and fine infant, which, like "a chip of the old block," imitates the parental motions, thrusts forth its cilia and works its gizzard.[3] In other genera the eggs are hatched externally, but this one is ovoviparous, and carries its nursery inside.
[3] This was met with in the summer, but is described here to avoid repetition. I do not know whether the eggs are hatched in very cold weather.