(100). Nearly all the Infusoria and Rhizopoda have in their interior a kind of nucleus, which is quite different in its compact texture from the parenchyma by which it is surrounded. The nucleus seems to play an essential part in the fissuration; for every time the animalcule divides either longitudinally or transversely, this organ, which is usually situated in the middle, divides also, so that each of the two new individuals has a nucleus. This nucleus, which is of a fine-granular aspect and dense structure, retains its form when the animalcule is pressed between two plates of glass and the other parts are spread out in various ways. By direct light its colour appears pale yellow.

(101). A remarkable mode of reproduction, by encystment, observed first by Stein in different species of Vorticella, appears to exist commonly among all the animalcules of this class*. The individual about to become encysted contracts slightly, and closes its peristome; and around it there then appears a cloud-like sacculus formed by a viscid liquid, which is probably the result of a cutaneous secretion. In this liquid are developed granules, which, augmenting more and more in number, and adhering together, finally form a membrane, which becomes hard and resisting, although soft and flexible when first produced. This encystment appears to have a double purpose: first, to withdraw these very delicate animals from the destructive effects of drought and cold; and secondly, to allow them to undergo certain metamorphoses protected from all external influences.

Vorticella microstoma (Ehr.), showing different stages of the process of fissiparous reproduction.

Fig. 21. Vorticella microstoma (Ehr.), showing different stages of the process of fissiparous reproduction. The basis to which the group is attached consists of a finely granular mucous mass.

* M. J. d'Udekem, "On the Metamorphoses of the Vorticellae," Ann. Nat. Hist, for July 1859.

The Vorticellian thus encased becomes attenuated and folded upon itself; the sarcodic substance appears to traverse its integument in all parts; from time to time it still contracts, but ultimately becomes completely dissolved, so that we find in its place merely a homogeneous sarcodic liquid, containing granules, together with the nucleus, which resists the general destruction. In the sarcode there takes place a process which may be in some measure compared with what occurs in the vitellus after the fecundation of an ovum. The granules becoming united together form groups, which soon divide and subdivide; at the same time an integument is formed upon the surface, which is contractile, covered with vibratile cilia, and closed at all points. The cyst now contains a new Infusorium, which may be compared with the Opalince or Bursarice which are met with in the intestines of the Ba-trachians.

(102). The next part of the process is the transformation of the ciliated Infusorium thus obtained into an Acineta. While the metamorphosed Vorticellian revolves in the interior of the cyst, it undergoes development, increases in size, and its whole surface becomes covered with folds: at length a moment arrives when the cyst, yielding to the pressure exerted in its interior, bursts; the ciliated Infusorium becomes free, and swims about with a rotatory movement, and is gradually developed into an Acineta. Fourthly, ciliated embryos make their appearance in the interior of the Acineta thus produced, apparently formed at the expense of the nucleus; the nucleus becomes totally converted into an embryo, and after the expulsion of the latter a new nucleus is formed, which in its turn becomes transformed into another embryo, and so on. Lastly, the embryos become fixed, and transformed into young Vorticellce.

According to Stein, therefore, the Vorticellce by this process of encysting are transformed into Acinetae, and these again, by means of internal motile embryos which are emitted from them, change into Vorticellae. But the reality of this metamorphosis has been disputed by several careful observers; nevertheless Professor Stein still retains his original views upon this subject*.

(103). The encapsulation of Kerona pustulata is thus described by Mr. Carter: - "The first change that occurred was the absence of all crude aliment in the abdominal cavity; then a division of the nucleus into four parts, preparatory to its disappearing altogether. At the same time certain dark angular grains, which had been floating round with the sarcode of the abdominal cavity, became congregated in the posterior extremity. The Kerona now became shortened, its cilia disappeared, and finally it passed into a rounded-oval ball. This, after a certain time, resolved itself into an obtuse-elliptical capsule enclosing a spherical cell with a separate mass of dark angular grains. The capsule was laminated and ragged on the outside, and defined by a clear line internally, while the spherical cell contained all the vital remains of the Kerona, together with the contracting vesicle, but exclusive of the 'dark angular grains,' which, adhering more or less together, were still enclosed within the capsule.

At this time the spherical cell was rotating, probably from the presence of cilia upon its surface, and the contracting vesicle active; but subsequently the granular mucus of which it was composed became transformed into a number of uniform, round, refractive, oil-looking bodies, and the contracting vesicle disappeared. After the lapse of nearly a month, the contents of the spherical cysts again became active, and had assumed definite forms, rotating rapidly in their cells, sometimes one way, sometimes the other, attended by intervals of rest; with many also a lifeless portion was present, which was forced round equally fast with the living one." Mr. Carter, however, was not fortunate enough to witness the actual escape of the enclosed animalcules.