THE members of this subkingdom are the lowest in the scale of animal organization, their bodies consisting of a soft gelatinous and structureless mass, which has a remarkable tendency to form little cavities or vacuoles in its substance, and is called sar´code (s???, flesh). They exhibit no organs, unless the cilia and certain variable processes formed of the common substance of the body, and which form their agents of locomotion, be considered as such,—this substance exercising the combined functions of motion, sensation, and secretion, for which separate organs exist in the higher animals.

Rhizop´oda (???a, root, p???, foot).—The animals belonging to this class consist of the structureless colourless substance to which reference has been made as sarcode, and they exhibit no organs. The sarcodic body is slowly contractile, and portions of it can be protruded at will in the form of irregular root-like processes, acting both as legs for locomotion and as tentacles by which the animal grasps its prey, which is then forced into the substance of the body, where it becomes surrounded by the surface, and a cavity is formed, within which it is digested.

Amoe´ba dif´fluens (Pl. XI. fig. 10) is common in water in which portions of plants have been kept for some time. When first placed on the slide, the body appears as a minute, transparent, rounded mass of jelly; but if observed for some time, it will be seen slowly to protrude its root-like processes; and foreign bodies, as DiatomaceÆ or other minute AlgÆ, will often be found imbedded in its substance.

Arcel´la vulgÁris (Pl. XI. fig. 6) is found among ConfervÆ in ponds and ditch-water. It is contained in a hemispherical shell or carapace, from the round orifice of which the lobed processes are protruded. The shell is covered with minute pits.

Arcel´la aculeÁta (Pl. XI. fig. 7) has the convex shell furnished with spines; fig. 8 represents the animal with its processes extended; while Arcella dentÁta (Pl. XI. fig. 9) exhibits an angular or somewhat toothed membranous shell. Both the latter species are met with in the same localities as the first.

ActÍnophrys sol (Pl. XI. fig. 12) is a very beautiful and excessively delicate Rhizopod. The body is spherical, and covered with very delicate and slender cilia-like processes. Its movements are exceedingly slow, and can only be observed by prolonged watching. The body appears to be reticulated, from the presence of numerous vacuoles.

Two large groups of genera and species of Rhizopoda, the animal bodies possessing the above general characters, mostly with very slender processes, exist, in one of which (the Foraminifera) they are contained in calcareous shells, often of elegant forms; while in the other (the Polycystina) the shells are siliceous or composed of flint, both kinds of shells being perforated with holes. These shells, which occur in the fossil state in enormous numbers, sometimes forming mountain-masses, are extremely beautiful objects for the microscope.

Spon´giÆ.—This class contains the Sponges, almost all of which are marine and foreign, and therefore not likely to come under observation in the perfect state. The substance commonly called sponge is the horny skeleton of the animal, consisting usually of rounded fibres (Pl. XI. fig. 13), irregularly netted and interlacing. The surface of a sponge exhibits minute pores and larger pouting orifices; the former of which admit currents of water, to be discharged at the latter, both being the mouths of continuous channels. The surfaces of the channels are lined with sarcodic matter, which takes the form of ciliated amoebiform bodies, by which the currents of liquid are produced.

The horny fibres of sponges are strengthened by little siliceous or flinty bodies of various forms (Pl. XI. fig. 13 a, b, c), which are imbedded in the substance of the fibres or attached to their surface, and form very curious microscopic objects. They are called spic´ula (spiculum, a dart), being often of a pointed form. In some sponges they are calcareous.

Infusor´ia.—The animals contained in this class are usually very minute, being rarely even perceptible to the naked eye, except when existing in very large numbers, so as to render the water milky, green, or red. They are found in all kinds of water, but especially in stagnant pools and in decomposing solutions or infusions of vegetable matters. The true structure of their bodies is a matter of doubt, some authors having considered them as being highly organized, while others have regarded them as consisting of simple cells; and whether they are correctly referred to the Protozoa must remain at present a matter of doubt. The body is of various forms, as represented in Plate XI. figs. 16-40. In some of them it consists of a simple sarcodic mass, evidently without any outer skin, as shown by its ready adhesion and laceration on accidental contact with foreign bodies; while in others the surface is regularly dotted with little depressions, or with nodules, so as to resemble a definitely organized structure.

The most striking character of the Infusoria is the presence of vibratile cilia, which are variously arranged; in some entirely covering the body, irregularly or in regular rows, in others being situated at definite parts only. By the action of the cilia they are enabled to swim freely in the water, also to obtain their food, which consists of minute AlgÆ or fragments of animal matter. In many of them there is a special row or set of cilia, which, by the currents it produces, urges the particles of food suspended in the water towards the mouth. The cilia also act as respiratory organs, by changing the water with which their bodies are in contact. In some of the species there are stout bristles or setÆ, by which they are enabled to crawl upon water-plants.

On carefully examining the bodies of the Infusoria, rounded granular spots will be seen, frequently containing minute AlgÆ, &c. (fig. 28 b). These spots are the digestive cavities, and have been called gastric sac´culi; but whether they are definite sacs or mere excavations, formed by the particles of food having been forced into the softer internal substance of the body, has not been positively determined. The sacculi may be filled artificially by mixing very fine indigo, or carmine, on a slide with the water in which the Infusoria are contained. A definite food-tube or alimentary canal has been detected in a few of the Infusoria; but it cannot be shown to exist in the majority of them.

A mouth exists in most of them, and is sometimes indicated by a row or set of cilia somewhat larger than those existing upon other parts of the body, and leading to or placed near it. The particles of food which have entered the body are often seen to pass round it, as if circulating, descending on one side and ascending on the other.

In addition to the gastric sacculi, certain clear transparent spots may also be seen within the body, appearing light or dark according to the adjustment of the focus. If these are attentively watched, they will be seen to contract and finally disappear, becoming again distended and vanishing at tolerably regular intervals. These are the contractile vesicles (figs. 27 a, 28 a, 37 a), and they contain a clear liquid, the nature of which is uncertain.

In many of the Infusoria is a round or elongate granular body (figs. 31 a, 36 a), which is called the nucleus, the term having been applied to it from a notion that the Infusoria consisted of simple cells. A minute red spot is also often seen at the anterior end of the body, which is supposed to represent an eye, and is called an eye-spot. The Infusoria are propagated in several ways:—by budding or gemmation, new beings sprouting out in a bud-like form, usually from the base of the parent; by division, either transverse (fig. 26 a) or longitudinal (fig. 34 b), of the body gradually into two parts, each of which subsequently becomes a perfect animal; by encysting, the body contracting into a globular form, and forming a firm coat around it, the contents becoming resolved into a numerous progeny of young; and by conjugation and the agency of spermatozoa and ova. We will now proceed to the examination of a few species, arranging them in the order of the families to which they belong.

Monad´ina.—In this family the bodies of the Infusoria are very soft, and without a skin or integument; they are also exceedingly minute, and will not admit the particles of indigo.

Mon´as lens (Pl. XI. fig. 16) is very minute, and commonly found in old infusions. Its body is rounded and flattened, and granular on the surface. At the front end of the body is a whip-like or flagel´liform (flagel´lum, a whip) filament, differing from a cilium in being rigid at the base and moveable at the end only, by which it is enabled to row itself through the water with a wriggling motion.

Cercom´onas glob´ulus (fig. 17) has a spherical body, with two flagelliform filaments, one arising from the front, the other from the end of the body. In Cercomonas crassicau´da (fig. 18) the posterior filament is replaced by a tail-like narrowing of the body.

Heterom´ita ovÁta (fig. 19) has the body ovate, with two long anterior flagelliform filaments, one of which is directed forwards, while the other trails behind.

Anthophy´sa mÜl´leri (fig. 20) has the monad bodies arranged in little heads at the ends of an irregularly branched brown stalk. After a time they become detached and revolve freely in the water.

Dinobry´ina.—Dinobry´on sertulÁria (Pl. XI. fig. 21) forms a minute Sertularia-like polypidom, consisting of rows of cells, each containing an oval monad with a single anterior filament. The two last species are common in bog-water.

Thecamonad´ina.—In these Infusoria the body is inclosed in a firm and sometimes brittle shell or carapace.

Trachelom´onas volvoc´ina (Pl. XI. fig. 22) has a spherical red shell, the body being furnished with a single filament and a minute red eye-spot; while ChÆtoglÉna volvoc´ina (fig. 23) has an oblong shell, covered with little spines.

EuglÉnia.—In this family the form of the body is constantly changing, being at one time spherical, at another fusiform or ovate. It is covered with a contractile skin or firmer external portion, and has one or more flagelliform filaments for locomotion. The species are common in stagnant pools, often colouring the water green or red.

EuglÉna vir´idis (Pl. XI. fig. 24) has a spindle-shaped body when fully expanded, the ends being pale; and at the front end is a red eye-spot.

AstÁsia hÆmatÓdes (fig. 25), which is probably a form of the Euglena, is found in stagnant pools, which it renders red. It has no eye-spot.

EnchÉlia.—These Infusoria are found in stagnant water and in decomposing infusions. The body is covered with cilia variously arranged, but there is no integument nor mouth.

En´chelys nodulÓsa (Pl. XI. fig. 26) has a colourless, oblong, irregularly nodular body, coated with very slender radiating cilia, and often exhibits numerous vacuoles. It is frequently found undergoing transverse division (fig. 26 a), the body becoming gradually constricted until it separates into two parts, which become perfect animals.

Alys´cum sal´tans (fig. 39) has an ovoid-oblong, slightly furrowed body, surrounded with radiating cilia, and has a side bundle of long retractile cilia, by means of which it leaps from place to place in the water.

KerÓnia.—In this family the body is soft, irregularly ciliated, without a special integument, but has an oblique row of vibratile cilia leading to the mouth, and stouter cilia or bristles (setÆ) on certain parts of the body. The sacculi often contain DiatomaceÆ, &c.

Oxyt´richa gib´ba (Pl. XI. fig. 27) has a colourless, oblong body, somewhat expanded in the middle, with setÆ at the two ends. In the side view (fig. 27 a), the body is seen to be convex above and flattened beneath.

Paramec´ina.—The species belonging to this family have a soft, flexible body, which is usually oblong and flattened beneath, with an integument covered regularly with pits and rows of cilia.

Col´poda cucul´lus (Pl. XI. fig. 30) has a slightly compressed body, ciliated all over, and kidney-shaped or rounded on one side and notched on the other, the surface exhibiting rows of nodules. The mouth is situated at the bottom of the notch.

ParamÉcium aurÉlia (fig. 28) has the body oblong or oblong-ovate, the mouth being placed near the anterior third of its under part. This infusorium is of comparatively large size, and is often found in immense numbers in infusions, which it renders milky. It is admirably adapted for showing the sacculi, which are easily filled with indigo. The body exhibits two remarkable stellate organs, consisting of a central contractile vesicle, surrounded by several radiately placed oval vesicles, which may be seen to contract and dilate with great regularity. The body is coated with very fine cilia.

Amphilep´tus fascÍola (fig. 29) is furnished with an elongate fusiform or lanceolate flattened body, with a lateral oblique mouth.

Chil´odon cucul´lulus (fig. 37) has an oblong thin body, irregularly wavy on the sides; the mouth being situated obliquely in front of the middle, and furnished with a cylinder of parallel rod-like teeth.

Nas´sula el´egans (fig. 31) has the body ovoid or oblong, becoming globular when contracted, the mouth being furnished with teeth as in Chilodon. It is often found among OscillatoriÆ.

UrceolarÍna.—Vorticel´la convallÁria (Pl. XI. fig. 34) is very commonly met with in decomposing infusions. The bell-shaped body is fixed at the end of a slender stalk, which is often seen to be extended and then suddenly contracted into a spiral (fig. 34 a). The cilia are arranged around a raised rim at the front of the body, and extend down a fissure leading to the mouth. The sacculi of this infusorium may be readily filled with indigo. The process of longitudinal division may also often be observed, taking about an hour for its completion; and when the new individual is about to separate from the parent, a ring of cilia may be noticed to have sprung up around the base (fig. 36). The encysting process is also often visible, the cilia disappearing, and the body becoming globular and secreting a cyst around it; after a time the contents become resolved into a number of embryos, which escape by the bursting of the cyst (fig. 35). In some cases the Vorticella assumes the form of a Podoph´rya (fig. 40), the surface becoming covered with tentacle-like processes. This Podophrya was formerly considered a distinct species.

Vaginic´ola crystal´lina (fig. 33) is contained in a crystalline tube, from which the body can be protruded. The body is of variable form, presenting when fully extended a trumpet shape. The cilia exist at the anterior end, and extend down a lateral fissure as in Vorticella. It is found attached to ConfervÆ in the water of ponds and bog-pools.

Sten´tor polymor´phus (fig. 38) is a very beautiful trumpet-shaped infusorium, the body being covered with spiral rows of cilia. The rim is furnished with stouter cilia, its margins at the notch being spirally turned inwards. This infusorium is often found in little groups attached to a gelatinous mass (fig. 38 c); and it is met with also in a free or unattached state.

Colep´ina.—CÓleps hir´tus (fig. 32) has a barrel-shaped carapace, transversely and longitudinally furrowed, the furrows being occupied by cilia. It has two or three spines behind, and ten or twelve at the front end of the carapace. It is common among ConfervÆ, and is very voracious, feeding upon dead Entomostraca, &c.; and if disturbed, at its meal by moving the cover, it will soon return and resume feeding as before.

Examination.—The Infusoria must be examined during life; for they are so altered by preservative liquids that they cannot be well preserved. The shells of those that are provided with them may be kept simply dried upon a slide, and in this way a few will retain their form, and the cilia of all may be more easily distinguished; the vacuoles may also then be seen very distinctly. When they are confined in a small quantity of water and are about to die, a curious phenomenon may be observed in them, a number of oil-like sarcodic globules exuding from the body, and within these, vacuoles may often be seen to form spontaneously.

The Infusoria may be collected in small phials; but it is difficult to keep them, as they form the food of the Entomostraca, the Rotatoria, and the larvÆ of insects; so that their enemies are very numerous, and they soon disappear.

Classification.—Before leaving the subject of living bodies, it may be well to make a few remarks upon their systematic relation as defined by classification.

All natural bodies are referable to one of three great kingdoms, viz. the Animal, the Vegetable, or the Mineral Kingdom. The bodies belonging to the latter seldom come under the notice of the microscopic observer, as they are mostly visible to the naked eye, and their minute structure is the same as that of the larger masses. The general structure of the members of the vegetable and animal kingdoms has been illustrated in the preceding pages. These bodies are distinguished from those of the mineral kingdom by their vital power of appropriating surrounding matters to their own nutrition and growth—this power being exercised by their organs, or, in the lowest forms, by any portion of their substance. Hence animals and vegetables or plants are termed organic bodies, while minerals are termed inorganic bodies, as having no organs; and the material of which organic bodies consist is termed organic matter, that of minerals being inorganic matter. But in both animals and plants inorganic matter is mixed with the organic matter, having been taken up or absorbed from the inorganic kingdom, although it does not usually exist in its characteristic condition, which is that of crystals, i. e. angular solids, as crystals of Epsom salts, &c. The individual members of the animal and vegetable kingdoms are systematically divided into certain groups, and these into successively smaller groups until we arrive at the species. These groups are founded upon the possession of certain points of resemblance by their members, forming the distinguishing characters, and their names are significant and definite. They usually run as follows, the larger and higher groups standing first in order:—Kingdoms, Subkingdoms, Classes, Orders, Families, Tribes, Genera, and Species. But it must be observed that the term species has a different value from that of the other terms; for the individuals of which the species consist are not only related by resemblance of structure, but also by their origin—being supposed to have derived their origin from a parent of original creation; while the other groups have, as far as we know, no other relation than that of similarity of structure.

It is obvious that the characters of the various groups might be founded upon peculiarities of any kind. But on this point two methods must be specially distinguished, in one of which the groups are founded upon the sum of all the peculiarities, while in the other they are based upon the structure of single parts or organs. In the former case the system is called natural, in the latter artificial. And while the latter often brings together beings which have perhaps but one or two points of resemblance, and separates others which are closely related, the former associates those which are really and naturally similar.

All the groups have special names, so that they may be referred to and spoken of as in the case of common things; the names being composed of Greek or Latin words, so that they may be intelligible to all nations; and as these are dead languages, they will remain good for all time.

In mentioning the name of an animal or plant, the name of the genus is always used with that of the species; thus, the name of Chickweed is Stellaria media. Because there are mostly several species in a genus; so that if the name of the genus only were used, the species meant would be uncertain; and as there are often species of the same name in different genera, if the name of the species only were used, the genus meant would be doubtful.

The classification of animals and plants serves two important purposes: one is that the structural peculiarities and affinities of the groups may be contrasted and a knowledge of their absolute and their differential characters acquired, and for this a natural system is eminently serviceable; the second is that of enabling any animal or plant to be simply distinguished from any other, for which an artificial or analytical system is extremely useful.