I Order Ciliata. This order comprises those Infusoria in which the outer layer of the body is more or less abundantly furnished with vibratile cilia, which serve either for locomotion or for the procuring of food. Besides cilia, properly so called, some of the ciliated Infusoria are provided with styles or jointed bristles, which are movable, and subserve locomotion; whilst others have little hooks or uncini, with which they can attach themselves to foreign bodies. As types of the order, Paramaecium and Vorticella may be selected, the former being free, whilst the latter is permanently fixed in its adult condition.

Paramaecium (figs. 31 and 32) is a slipper-shaped animalcule, composed externally of a structureless transparent pellicle - the "cuticle" -which is lined by a layer of firm and consistent sarcode, which is termed the "cortical layer," or the "parenchyma of the body," this in turn passing into a central mass of softer and more diffluent sarcode, known as the "chyme- mass," or "endoplasm" The cuticle is merely the structureless hardened external lamina of the "cortical layer," and it may in some cases form a regular protective sheath (Vaginicola), a horny shell (Codon-ella), or even a reticulated siliceous envelope (as in Dictyocysta). Beneath the "cuticle" is the layer from which the cilia are given off, and below that, again, is a finely striated or fibrillated contractile layer ("myophane layer" of Haeckel), which corresponds physiologically to the muscles of higher animals. In some Infusorians there is a still more internal lamina of the "cortical layer," which is charged with the singular little organs known as "trichocysts." These are vesicular microscopic bodies, capable of emitting thread-like filaments, in many respects closely resembling the "thread-cells" of the Caelenterata. The "cuticle" in Paramaecium is covered with vibratile cilia (figs. 31 and 32), and is perforated by the aperture of the mouth.

Fig. 31.   Paramaecium, viewed dorsally, and greatly magnified. m Mouth; m to g Gullet; a Anus ; cv' and cv The contractile vesicles; I, II, III, Canals proceeding from the anterior contractile vesicle; n Nucleus; v Large cilia bounding the depression (

Fig. 31. - Paramaecium, viewed dorsally, and greatly magnified. m Mouth; m to g Gullet; a Anus ; cv' and cv The contractile vesicles; I, II, III, Canals proceeding from the anterior contractile vesicle; n Nucleus; v Large cilia bounding the depression ("vestibule") leading to the mouth. The arrows indicate the course in which the particles of food circulate in the semi-fluid protoplasm of the interior of the body. (After James-Clark.)

I Order Ciliata 41I Order Ciliata 42Fig. 32.   A, Paramecium, showing the nucleus (n) and two contractile vesicles (ji). B, Paramecium bursaria (after Stein) dividing transversely: n Nucleus; n' Nucleolus ; v Contractile vesicle. C, Paramecium aurelia (after Ehrenberg), dividing longitudinally.

Fig. 32. - A, Paramecium, showing the nucleus (n) and two contractile vesicles (ji). B, Paramecium bursaria (after Stein) dividing transversely: n Nucleus; n' Nucleolus ; v Contractile vesicle. C, Paramecium aurelia (after Ehrenberg), dividing longitudinally.

The mouth leads into a funnel-shaped gullet, which is not continued into a distinct digestive sac, but loses itself in the soft central protoplasm. On the line of boundary between the cortical layer and the diffluent central sarcode are placed the "nucleus" and the "contractile vesicle" (or vesicles). The "nucleus" is an oval body (in some forms band-shaped or rodlike), consisting of an outer membrane enclosing granular contents, and often having a smaller spherical particle applied to its exterior or immersed in its substance. This latter is the so-called "nucleolus," which must be carefully distinguished from the nucleolus of a cell, which occurs in the interior of the nucleus. The contractile vesicles are clear spaces, which contract and dilate at intervals, and occasionally exhibit radiating canals passing into the surrounding sarcode. Ordinarily one contractile vesicle is present, or at most two, but in some cases there may be several. It has also been maintained that the contractile vesicles communicate with the exterior of the body, but proofs are wanting on this point. Whether this should ultimately be established or not, there can be little doubt but that the vesicles are a rudimentary form of vascular apparatus. Others, however, hold, with some probability, that the contractile vesicles are to be regarded as excretory in function, and that they correspond more with the water-vascular system of the Scolecida than with the true blood-vascular system of higher animals. Certain other spaces termed "vacuoles" are generally visible in addition to the contractile vesicles. These, however, are probably merely collections of water surrounding the particles of ingested food, and performing with them a circulation in the abdominal cavity, something like the circulation of granules which is seen in certain vegetable cells. It was the appearance of these "vacuoles" - which are certainly not permanent organs of any kind - which induced Ehrenberg to term the Infusoria the "Polygastrica," upon the belief that the vacuoles were so many stomachs.

Paramaecium obtains its food by means of the currents of water which are set up by the constantly vibrating cilia. The nutritive particles thus brought to the mouth pass into the central abdominal cavity, along with the contents of which they undergo the circulation above spoken of. Indigestible and faecal particles appear to be expelled by a distinct anal aperture, which is situated near the mouth.