With Figure 13.

Paramecium Aurelia, the Slipper Animalcule (Fig. 13, A. B. and 1, 2, 3, 4), is commonly found in pond water and vegetable infusions. The length of its body varies from1/120 to 1/96 of an inch. It is more or less asymmetrical in shape, and one surface is slightly convex, the other flat. As the latter is pierced by the pharynx it is generally termed ventral. The exact outline varies somewhat with the state of contraction of the animal.

The surface of the body is covered by a very delicate cuticle (B: cu.) secreted by the underlying protoplasm and pierced by pores through which pass the cilia. Paramecium is holotrichous, i. e. the cilia covering the body are equal in size. There are larger adoral cilia leading to the pharynx, as there are in some Holotricha. The cilia (A: B: ci.) are disposed in longitudinal lines on the surface of the body (A.) and extend into the pharynx (B: p.), in which they create a current, up one side and down the other. They are, therefore, not only organs of locomotion but also of alimentation. They are structurally filamentous extensions of the cortical layer of the protoplasm, and undergo alternate movements of flexion and erection, which commencing at one pole of the organism, are propagated in successive waves to the other pole. The protoplasm which makes up the whole substance of this unicellular animal is divisible into two parts, an external cortical layer (B: C.) surrounding a central medulla (B: M.). These two parts are sharply contrasted. The cortex is contractile, dense, hyaline, and of great refractile power, while the medulla is fluent, soft, more or less granular, and opaque.

The cortex is marked in many Infusoria, but not evidently in Paramecium, by alternate light and dark lines, e. g. in Stentor. The former corresponds to lines in which the cortex is thicker, and hence contraction of the body takes place in a direction coincident with their length. The cortical substance is doubly refractile, and hence depolarises light. This fact is especially evident wherever it is thickened.

A. and B. Paramecium: from Ray Lankester, op. cit. infra, Fig. xxv. 1 and 2. 1, 2, 3, 4. Formation of vacuole: (after the same Fig. xxv). C. Amoeba Proteus: from Leidy, op. cit. infra, PI. i. Fig. 4.

Fig. 13. A. and B. Paramecium: from Ray Lankester, op. cit. infra, Fig. xxv. 1 and 2. 1, 2, 3, 4. Formation of vacuole: (after the same Fig. xxv).-C. Amoeba Proteus: from Leidy, op. cit. infra, PI. i. Fig. 4.

The cortex contains the trichocysts, contractile vacuoles, nucleus, and paranucleus. The trichocysts (B: t.) are not present in all Infusoria. They are numerous in Paramecium, and form a superficial layer beneath the cuticle. They are minute sacs containing a simple spirally coiled filament which is eversible (B: tf.), and therefore they closely resemble the nemato-cysts of many Coelenterata. The contractile vacuoles are two in number, one in front of the other. Saville Kent states that under normal conditions they are evenly spheroidal in outline: but that under slight pressure they are formed in a manner normal to certain types and exemplified in Fig. 13, 1, 2, 3, 4. Minute pyriform drops make their appearance pointing towards a common centre. They enlarge, and their inner ends approaching give origin to a central drop which swells as the radiating spaces disappear and bursts externally, discharging its contents completely and disappearing in its turn. During the process of transverse fission two vacuoles make their appearance in the anterior region of the body, forming the vacuoles of one of the two new individuals, the other retaining the two previously existent vacuoles.

The vacuoles are constant in position: their function is to discharge superfluous water, containing perhaps soluble excretory products. It has been observed by Mr. A. G. Bourne that when the organism is fed on food-particles stained with aniline blue soluble in water, the dye is rapidly excreted by the vacuoles in a concentrated form.

The nucleus or endoplast (B: n.), and paranucleus or endoplastule (B: n'.), sometimes erroneously termed nucleolus, lie in a thickening of the cortex. In some instances they lie in the medulla and circulate with its movements. They were at one time regarded as ovum and testis respectively. In fission and during conjugation they divide, in fission once, in conjugation twice at least: and in the latter case the segments of the twice divided nucleus are usually further broken up. During the process of division nucleus and paranucleus alike have been observed to become striated. After conjugation, a process which is only temporary in Paramecium, and in which protoplasm is certainly exchanged between the conjugating individuals, both structures are reconstituted. The new nucleus is said to be formed by the fusion of two portions of the twice divided paranucleus. Some of the fragments of the nucleus are stated to be expelled, while the remaining fragments of both structures disappear unless a few of them fuse to form a new paranucleus. But in Stylonychia this body has been observed to arise by fission from the nucleus. The facts stated lead to the opinion that the paranucleus is nothing more than a small nucleus.

Paramecium is therefore bi-nucleate. Paranuclei are commonly found in Infusoria; but in some multinucleate forms, as e.g. Opalina, the nuclei are not only numerous but alike in size and other characters.

The medulla (B: M.) forms the central portion of the body. It is digestive in function and receives food-particles through a pharynx or tubular prolongation inwards of the cuticle (B: p.). A groove commencing on the left side of the animal leads obliquely to the entrance of this pharynx. At its base is the cell-mouth or cytostome (B: o.), where the medullary protoplasm is bare. The food-particles collect here in a minute drop of water (B: w.). Water and food enter the medulla, where they circulate together as food-vacuoles (B: v.), in the direction shown by the arrows in the figure. The water in these vacuoles is slowly absorbed, while the nutritive portion of the food is removed by a process not understood. In some instances an acid reaction has been observed in the inclosing water. The particles that remain are faecal, and, according to Saville Kent, are expelled midway between the mouth and posterior extremity of the body. It is not clear that Paramecium possesses a cell-anus or cytopyge lined by cuticle such as exists in some forms. The granules in the medulla are albuminoid and fatty in nature.