The Noctiluca is propagated by spontaneous division, a line appears bisecting the globe, which becomes more and more constricted till the animal is like a dumb-bell; the slender thread separating the two parts is then broken by their efforts to get free; the two new creatures swim off in different directions, and soon assume their adult form. But in many individuals there are clear, yellow globules with a well-defined nucleus, of a rich reddish-brown, which are the germs of the animal.
Most of the Infusoria multiply by continuous bisection, like the unicellular Algæ. The division generally begins with the nucleus, and is longitudinal or across, according to the form and nature of the animal, and is accomplished with such rapidity, that, by the computation of Professor Ehrenberg, 268,000,000 of individuals might be produced from one single animalcule of the species Paramœcium in a month. The Paramœcia are reproduced too by gemmation, and, as they are male and female, they are reproduced also like the higher classes.
The Infusoria have another mode of increasing. The animalcules either draw in or lose their cilia, and consequently come to rest. The animal then assumes a more globular form, and secretes a gelatinous substance from its surface, which hardens into a case or cyst, in which its body lies unattached and breaks up into minute ciliated gemmules, which swim forth like zoospores as soon as they come into the water by the thinning away of part of the cyst. In fact the animal is resolved into its offspring, which, as soon as free, gradually acquire the parent’s form, though at first they may bear no resemblance to it. The scarlet Peridium seen by Mr. Carter in the Bay of Bengal is propagated in this manner. For the parent Peridium is broken up within its cyst into from two to four new ones, each of which when set free and grown up might undergo the same process.
The Loxades bursaria increases by three distinct methods, and sometimes by two at a time. In autumn, or the beginning of winter, six or eight germs containing granular matter and one or more hyaline nuclei are formed within the animal, each enclosed in two contractile cysts: they lie freely in the cavity of the body, and come one by one into the water through a canal ending in a protuberance in the skin. During this time the pulsations of the vesicles within the Loxades are continued, but the gyration of the green particles is suspended till all the germs are excluded and swim away, and then it is renewed as vigorously as ever. At first the young are totally unlike their parent, but by degrees acquire its form. The Loxades is also increased by division, sometimes across, sometimes longitudinally, and, in the latter case, one half is occasionally seen to contain germs which have been excluded before the other half had separated, so that the two distinct systems of propagation are simultaneous.[20]