Paramecium aurelia (Ehr.), magnified 300 diameters.

Fig. 18. Paramecium aurelia (Ehr.), magnified 300 diameters. 1. The animalcule at rest, under slight compression: o', the position of the mouth; 8,8, contractile vesicles surrounded with radiating processes; t, nucleus. 2. Another specimen, placed in water coloured with indigo, showing the ciliary currents, the oral aperture, o', and the coloured pellets in the interior of the body. 3. The same in process of longitudinal division, each segment containing a distinct nucleus. 4. A specimen swimming freely about after being fed with carmine; the mouth, nucleus, and star-like contractile vesicles are seen more in profile.

(88). Radiating branches of the contractile spaces were first discovered by Ehrenberg in Paramecium and some other Infusoria. When the contractile space is fully expanded, the rays can only be observed as fine lines, or, when the light is not good, are entirely imperceptible. On the sudden contraction of the space, however, they instantly swell into pyriform cavities placed close to the position of the contractile organ which has disappeared. During the slow reappearance of the contractile space, the rays gradually decrease, and they in turn have almost disappeared or become reduced to fine lines when the vesicle has attained its full extension. These rays, as well as the contractile spaces, lie, as in all Infusoria, close under the skin ("cuticula" of Cohn), in the parenchyma of the body ("cortical layer" of Cohn).

(89). The processes of the contractile space are seen with remarkable clearness in the large Stentor polymorphus, in which a very considerable portion of a vascular system may be recognized. The contractile space lies a little to the left of the oesophagus, near the plane of the ciliary disk; from it a longitudinal vessel runs to the posterior extremity of the animal, and an annular vessel round the ciliary disk, close under its series of cilia. Both these are visible even during the expansion of the contractile vesicle, but swell up suddenly during its contraction, like the rays of Paramecium above described; and then the longitudinal vessel usuallv!exhibits considerable dilatations, which, when superficially examined, might easily be taken for independent disunited cavities (vacuoles): the annular vessel exhibits a more uniform aspect; only two rounded dilatations make their appearance in it. Both vessels gradually decrease during the reappearance of the contractile vesicle, apparently without any contraction of their own.

It is therefore probable that in all the Infusoria possessed of a contractile space, it is the centre of a vascular system which does not merely consist of chasms formed in the parenchyma by its accidental separation. Another and more difficult question concerning its nature remains to be cleared up, namely whether the vessels and the contractile space possess proper walls - in other words, whether the contractile space is or is not a vesicle. The mode of its contraction, which differs from the other contractile phenomena of the parenchyma of the body, appears to speak decidedly in favour of its vesicular character, as do some other facts, such as the phenomenon presented by Spirostomum ambiguum, an animalcule in which the anus is situated at the hinder end of the animal, and close in front of it is the very large contractile space. When fully expanded, this space appears to be surrounded only by a thin membrane; but nevertheless pellets of excrement, often several at the same time on different sides of the vesicle, form projections which are nearly hemispherical both towards the vesicle and the outer surface of the body, - a circumstance which could not happen if it were not a vesicle with proper walls.

Stentor Mulleri (Ehr.), magnified. It is represented in a half contracted state.

Fig. 19. Stentor Mulleri (Ehr.), magnified. It is represented in a half-contracted state, and placed in indigo-water, so as to show the ciliary currents.

(90). When examined with a good microscope, the Infusoria are found to be possessed of another organ of very mysterious character, which, in accordance with the prevailing notion of their analogy with animal or vegetable cells, has been named the nucleus, and which certainly, as we shall see hereafter, seems to play an important part in their economy, more especially in connexion with the propagation of these animalcules.

The nucleus is usually roundish or longish, or even (as in many Vor-ticellince and Stentor) much elongated and band-like. It is enveloped in a peculiar membrane, and generally presents a homogeneous or finely granular appearance. It appears constantly to enclose a cavity surrounded with very thick walls, and sometimes contains a smaller body, designated nucleolus, which, however, in certain species is situated external to the nucleus.

The nucleus, as a general rule, seems to be affixed to the walls of the animalcule, as it does not appear at all to participate in the rotatory movements observable in the interior of the body.

(91). In most, if not all Infusoria, the whole surface of the body is capable of exuding a gelatinous matter. Some do this regularly; and the exuded material either retains its gelatinous consistence, or, hardening into a horny substance, forms a sheath or shell (lorica), into which the animalcule can contract itself more or less completely.

Another kind of exudation also occurs in a great many species, which leads to the formation of a case or cyst enclosing the animalcule that secretes it - the object of which appears to be the protection of the encysted animal from unfavourable circumstances in the surrounding-water, and from death from desiccation. This process of encystation is sometimes connected with the reproductive function, as we shall see hereafter.