STRUCTURE OF PROTOZOA. 135 polypes; and new branches, are formed either by the subdivision of the polypes, or by gemmation from the connecting substance. 128. When we pass from Zoophytes to animals of still simpler organization, we lose all trace of definite symmetry, and find ourselves amid forms which cannot be referred to any particular plan of growth. These, moreover, are for the most part distinguished by an extreme simplicity of structure; no such differentiation of parts exhibiting itself among them, as is shown in the "organs" of even the simplest Zoophyte or Worm. Hence they are appropriately designated PROTOZOA. They may, in fact, be considered as essentially consisting of homogeneous particles of a jelly-like substance, to which the name of Sarcode has been given; and the chief modification this undergoes, consists in the consolidation of certain parts of it by the deposit of horny, calcareous, or siliceous matter, so as to form a skeleton. This may take place on the outer surface only, so as to form shells very like those of Mollusks in miniature, as we see among Foraminifera (fig. 78); or it may occur in the midst of the fleshy substance, so as to form an internal network, such as presents itself in the Sponge. The endowments of the "sarcode" are very extraordinary; and will be best understood by observation of the life-history of one of those simplest Protozoa, in which the whole body consists of but a minute particle of it. B Fig. 77.-RHIZOPODA :—A, Amoeba; B, Actinophrys. 129. Such an example is afforded by the Amoeba (fig. 77 a), a creature frequently to be met with in great abundance in fresh and stagnant waters, vegetable infusions, &c. Its 136 RHIZOPODA :-AMŒŒBA; ACTINOPHRYS. organization is so low, that there is not even that distinct differentiation into containing and contained parts which is necessary to constitute a cell (§ 32); for although the superficial layer of the sarcode possesses more consistence than the interior, it is nevertheless obvious that it has not the tenacity of a membrane, since (as will be presently seen) it does not oppose the passage of solid particles into the interior. However inert this creature may seem when first glanced at, its possession of vital activity is soon made apparent by the movements which it executes and the changes of form it undergoes; these being, in fact, parts of one and the same set of actions. For the shapeless mass puts forth one or more finger-like prolongations, which are simply extensions of its gelatinous substance in those particular directions; and a continuation of the same action, first distending the prolongation, and then, as it were, carrying the whole body into it, causes the entire mass to change its place. After a short time another prolongation is put forth, either in the same or in some different direction; and the body is again absorbed into it, so as to shift its place still more. It is by means of this movement that the creature obtains its supplies of food; for when, in the course of its progress, it meets with a particle appropriate for its nutriment, its gelatinous body spreads itself over this, so as to envelope it completely; and the substance (sometimes animal, sometimes vegetable), thus taken into this extemporized stomach, undergoes a sort o digestion therein, the nutrient material passing into the substance of the sarcode, and any indigestible portion making its way to the surface, from some part of which it is (as it were) finally squeezed out. 130. Many other forms of this group, which has received the designation of Rhizopoda, have less power of moving from place to place, but obtain their food by a modification of the same arrangement: of this we have an example in Actinophrys (fig. 77 B). The body being stationary, its gelatinous substance extends itself into long filaments, termed pseudopodia: these often divide themselves again like the roots of a tree (whence the designation of the group), so as to form threads of extreme tenuity; and sometimes these threads meet again and coalesce, so as to form a sort of irregular network. When any minute animal or vegetable organism happens to come in contact RHIZOPODA -FORAMINIFERA. 137 with one of these threads, it is usually held by adhesion to it, and the filament forthwith begins to retract itself; as it shortens, the surrounding filaments also apply themselves to the captive particle, bending their points together, so as gradually to inclose it, and themselves retracting until the prey is brought to the surface of the body; and the substance of the threads being itself drawn into that of the body, the entrapped particle is embedded along with this, and undergoes digestion in the surrounding sarcode, any indigestible particle being subsequently extruded from the surface of the body, just as in the Amoeba. The reproduction of these creatures, so far as is yet known, is effected by self-division, like that of the Infusoria (§ 135); but there is reason to believe that a "conjugation," or reunion of two individuals, sometimes occurs, and that this is to be looked on as representing the sexual propagation of higher animals. A, Oolina; B, C, Nodosaria; D, Cristellaria; E, Polystomella; F, Dendritina, G, Globigerina; H, Textularia; I, Quinqueloculina. 131. This Rhizopod type of animal life is manifested in two groups of great interest, which are characterised by the possession of hard shells, formed by the consolidation of the external layer of sarcode. The Foraminifera have calcareous shells, which often bear a strong resemblance to those of Nautili, &c. in miniature (fig. 78), but which really have an entirely different relation to the animals that form them. For whilst the Nautilus occupies only the last or outer chamber of its shell, the chambers previously formed 138 FORAMINIFERA AND POLYCYSTINA. being empty and deserted, each chamber of the Rotalia, or any other Foraminiferous shell, is occupied by a segment of sarcode, which is to a great degree independent of the rest, and is only connected with those on either side of it by delicate threads of the same substance; and the extension of the shell is due to the formation of an additional segment of sarcode on the outside of the last-formed chamber. Each segment has usually the power of putting forth its own "pseudopodia" through minute apertures in the shell, and thus of drawing in its own nourishment through these; but even when (as sometimes happens) these food-collecting threads are put forth from the last chamber alone, the nutriment there obtained is transmitted to the segments within by percolation through the substance of the sarcode, and not through any tubular canal.-The accumulation of the shells of Foraminifera in some parts of the existing sea-bottom is very remarkable; and similar accumulations in past ages have formed no unimportant part of the crust of the eartha large part of the Chalk-formation having had its origin in them, as well as nearly the whole of the Nummulitic limestone by which it was succeeded. A B 132. But animals whose essential structure seems to be nearly the same, may form siliceous instead of calcareous shells; and thus are produced those beautiful organisms, known under the name of Polycystina (fig. 79), which are occasionally found in the existing seas, but whose remains are met with under a far greater variety of forms in certain of the newer marine deposits. There is not in these the same tendency to form composite structures by the multiplication of segments, as in the Foraminifera; but the complication of the individual form is often much greater. Yet, however complex the form, the essential composition of these creatures seems to retain the same attribute of simplicity, which cannot be conceived capable of further reduction. M Fig. 79.-POLYCYSTINA. A, Podocyrtis; B, Rhopalocanium. INFUSORY ANIMALCULES. 139 133. The Animalcules to which the name of INFUSORIA may be properly restricted (the Rotifera, or Wheel-Animalcules, § 105, whose organization is much higher, together with many organisms whose true nature is vegetable, being excluded), present an advance upon the simplicity of the Rhizopoda in this, that whilst their bodies consist for the most part of sarcode, and present scarcely anything that can be termed a distinction of organs, their external surface is condensed into a membrane too firm to admit either of indefinite extension into pseudopodia, or of the passage of alimentary particles through it; and consequently the form of the body, although not insusceptible of being temporarily changed by pressure, possesses a considerable degree of constancy for each species (fig. 80). A mouth, or definite aperture for the ingestion of food, is provided; with an additional orifice in some instances, through which indigestible or effete matters may be discharged from the interior. Into this mouth, ali Fig. 80.-INFUSORY ANIMALCULES. 1. Monads; II. Trachelis anas; III. Enchelis, discharging fæcal matter, Iv. Paramœcium; v. Kolpoda; vi. Trachelis fasciolaris. mentary particles are drawn by the agency of the cilia with which some part of the surface of the body is provided; these cilia being always so disposed as to serve at the same time for the general locomotion of the animalcule, and for the production of currents that shall bring food to its interior. 134. Although most Infusoria move freely through the water in which they live, yet certain kinds of them attach themselves by footstalks to marine plants or other floating bodies, during at least a part of their lives; and in this condition bear no slight resemblance to Zoophytes, though of far simpler organization. It is in these sessile forms that the agency of the cilia in creating currents which bring food to |