Morphology of Paramecium | Protozoa | Microorganisms | Zoology

In this article we will discuss about the morphology of paramecium.

Size and Shape:

P. caudatum is a microscopic organism and is visible to the naked eye. It is light grey or white in colour and commonly measures 170 to 290 microns. The animal appears like the sole of a slipper and is therefore known as slipper animalcule. The body is long, blunt and rounded at the anterior end and somewhat pointed at the posterior end. The widest part of an animal lies just after the middle end. The body of the animal is asymmetrical in form having an oral or ventral surface and an aboral or dorsal surface.

Pellicle:

The body is covered by a thin, double layered, elastic and firm pellicle made of gelatin. The pellicle holds the shape of the animal but is elastic enough to permit contractions. The pellicle has double membrane, the outer membrane is continuous with the cilia and the inner membrane with the ectoplasm.

Under the higher magnification of microscope, pellicle shows rectangular or hexagonal depressions on its surface. This arrangement is preserved on the dorsal surface of Paramecium but on the ventral surface the ridges converge in front and behind towards a preoral and postoral aperture.

Cilia:

The entire body is covered by numerous, tiny hair-like fine projections, the cilia. They lie longitudinally and are equal in size. Such a condition is known as holotrichous. Each cilium is about 10-12 µ in length and 0.27 µ in diameter.

It has an outer protoplasmic sheath or plasma membrane with nine double longitudinal fibrils in a peripheral ring. There are about 10,000 to 14,000 cilia covering the body. There are a few larger cilia at the posterior end forming a caudal tuft. Cilia are also associated with the mouth region.

Under high power, the cilium is found to be similar to flagellum and consists of a fluid matrix surrounded by an outer membranous sheath continuous with the outer body membrane. Within the matrix are nine peripheral longitudinal fibres which run along the whole length of the cilium.

Each fibres is formed of two sub-fibres. In the centre of the matrix are two single fibres which are enclosed within an inner membranous sheath. In between the central and peripheral fibres are nine additional accessory fibres.

The base of each cilium is produced into a tube like basal body or kinetosome. It is formed by the thickened basal ends of the peripheral fibres of the cilium. The wall of the basal body consists of nine triplets of sub-fibres. The basal bodies are self-duplicating and progenitors of new cilia.

Associated with the basal bodies of the cilia and lying in the ectoplasm is a system of specialized striated fibres called kinetodesmal fibrils. A single kinetodesmos arises from the kinetosome of each cilium and runs anteriorly. It joins its counterpart from the posterior kinetosomes, forming a bundle of overlapping longitudinal fibres called kinetodema, having 5 fibrils each.

It is supposed that the fibres coordinate ciliary beat and movement. The kinetosomes of a longitudinal row plus their kinetodesmata constitute a structural unit, the kinety—characteristic of all ciliates. The kinetia lie in the cortex beneath the pellicle. The kinetia bring about formation of organelles such as mouth during cell division.

Oral Groove:

On the ventrolateral side is a large oblique, shallow depression called oral grove or peristome which gives the animal an asymmetrical. Each hexagonal depression is perforated by a central aperture through which a single cilium emerges out. The anterior and posterior margins of hexagonal depressions bear the openings to trichocysts.

The electron microscopic study of pellicle by Ehret and Powers (1957) has revealed that the hexagonal depressions correspond to regular series of cavities the alveoli. All alveoli collectively form a continuous alveolar layer, which is delimited by an outer alveolar and inner alveolar membrane. The outer layer lies in close contact beneath the outer cell membrane. Therefore, pellicle includes outer cell membrane, outer alveolar membrane and inner alveolar membrane.

Electron microscopic studies by Ehert and Powers (1959) reveal a complicated structure of pellicle in Paramecium. In fact it consists of three membranes, the outermost being the cell membrane continuous over the body surface and cilia. The other two membranes are the outer and inner membranes, enclosing between them a series of closely packed spaces, called alveoli, which collectively form a continuous mosaic alveolar layer.

The polygons or hexagons, seen from outside, in fact, correspond with the internal alveoli which are inflated kidney shaped. It runs obliquely backwards from one side (usually left to right but in some cases right to left) and ending a little behind the middle body. The oral groove leads into a short conical funnel-shaped depression called vestibule. The vestibule leads directly into the fixed, oval-shaped opening called cytostome (mouth).

Extending directly from the cytostome toward the centre of the body is the wide cytopharynx. The cytopharynx then turn sharply towards the posterior side to become the slender tapering oesophagus. Thus, the oesophagus is roughly parallel to the body surface of Paramecium except at its posterior extremity. Here the oesophagus turns again toward the centre of the animal to lead into the forming food vacuole.

The ciliation of cytopharynx is very complicated. A structure called penniculus is found on the left wall of the cytopharynx and spirals through approximately 90 degrees so that its posterior extremity is on the oral (ventral) surface of the oesophagus. According to Lund, the penniculus consists of eight rows of cilia arranged in two closely set blocks of each.

A similar band composed of four rows of long cilia which are less compact than in the penniculus is termed as quadrulus. It spirals down the dorsal wall of buccal cavity and ends close to the penniculus. The penniculus and quarulus have been wrongly called an undulating membrane by some workers.

The quadrulus and penniculus control the passage of food. It is not known how cilia work, probably their fibrils contract in rhythmic way which causes bending. Gelei (1925) pointed out that the function of penniculus is the forcing of food elements into the body.

Cytoplasm:

The cytoplasm is divisible into narrow, external cortex or ectoplasm and broader, internal medullary region or endoplasm. The ectoplasm forms the firm, clear, thin and dense outer layer. It bears the trichocysts, cilia and fibrillar structures and is covered over by pellicle.

Trichocysts:

Trichocysts are embedded in the ectoplasm alternating with basal bodies lying at right angles to the body surface and open by small pores on the ridges of the hexagonal areas of the pellicle. Each trichocyst is very small being about 4 μ long. It consists of an elongated shaft and a terminal tip called the spike covered by a cap.

The matrix of shaft consists of dense mass of fibrous protein known as trichism. The trichocyst is filled with a refractive, dense fluid. The trichocysts arise from kinetosomes of cilia, then migrate and lie at equal distance in the endoplasm. When the animal is irritated, the trichocysts are discharged due to mechanical, chemical or electrical stimulation. When fully discharged the shaft becomes a long cross-striated rod and measures about 40 µ in length.

The function of trichocyst is not well understood but it is believed that they serve as organelles of defence. It is also said that they help the animal to fix on a sport during feeding. They are also supposed to be the organs of offence. After trichocysts are discharged, new ones are regenerated from kinetosomes.

Kinety:

The infraciliature is formed by compound morphological units, called kineties. A single unit or kinety is constituted by a longitudinal row of kinetosomes and their fibrils (kinetodsmata). A kinety system is characteristic of ciliates in general, playing important role in their morphogenesis. In Paramecium, a new mouth will fail to develop if the set of kineties, responsible for it, is removed experimentally.

Neuromotor System:

According to light microscope, beneath the pellicle of Paramecium there are also present ectoplasmic fibrils, other than kinetodesmata, with no surface striations. These directly connect together the kinetosomes and an internal fibrillar network with a darkly staining conspicuous, bilobed body, the motorium, lying near the cytopharynx, and probably, also with the trichocysts. According to Rees (1922) and Lund (1933), motorium, together with the associated fibrillar network, forms a neuromotor system.

It extends throughout the ectoplasm. The true nature of this system is not properly understood. Regarded sensory and analogous to the nervous system of higher animals, it is supposed to co-ordinate the ciliary movements. The other functions attributed to it are contractility, conductivity, elasticity and mechanical support.

According to electron microscopic studies, the alveoli, kinetosomes and kinetodesmata make their presence and there is no evidence regarding the existence of any neuromotor system in Paramecium.

Endoplasm—or medulla is more fluid and has many cytoplasmic granules and other inclusions such as mitochondria, Golgi bodies, crystals, granules, chromidia, nuclei, contractile vacuoles and food vacuoles.

Nuclei:

In the endoplasm are embedded two nuclei. They differ in size and appearance, and also in function. The smaller one of the micronucleus is lodged in a depression at one side of the larger nucleus called the meganucleus or macronucleus. The mega-nucleus is a conspicuous kidney-shaped body lying approximately in the centre of the body.

It is smooth and generally regular in outline. It is a compact structure containing fine threads and tightly packed discrete chromatin granules of variable sizes. It usually divides amitotically. The micronuclei are smaller structures and may be compact (P. caudatum) or vesicular (P. aurelia).

Their number varies in different species. P. caudatum and P. bursaria, etc. have one compact ellipsoid micronucleus. P. aurelia two spherical vesicular micronuclei and P. multinuicronucleatum have 3-9 spherical vesicular micronuclei. Because two types of nuclei are present the paramecia are said to possess dimorphic nuclei. The meganucleus seems to control the ordinary “vegetative” activities of the animal while the micronucleus is concerned with reproduction.

Contracile Vacuole:

In the endoplasm are food vacuoles of various sizes containing material undergoing digestion, whereas, towards each end of the cell-body there is a large clear contractile vacuole. Normally paramecia possess two contractile vacuoles (except P. multi. micronucleatum in which the number varies from 2-7). Contractile vacuoles are situated directly underneath the ectoplasm on the dorsal surface.

The anterior vacuole is sometimes called the nuclear vacuole being near the nucleus and the posterior one is called the peristomial vacuole being the vicinity of the peristome. In the vacuoles of Amoeba and Euglena smaller accessory vacuoles coalesce to form a definitive contractile vacuole as such each is termed vesicle-fed vacuole.

In paramecia the vacuole is fed by canals and is called the canal fed vacuole. The number of canals varies from 1 to 10 although the common number is 5 to 7. They are slender and radiating structures found in one plane, and form a characteristic rosette about each larger roughly spherical vacuole and empty into it.

They are called feeders or feeding canals. The functional cycle of the vacuole is simple. A vacuole becomes enlarged with fluid (mainly water) until it reaches the maximum size (diastole), then it suddenly collapses (systole) discharging the vacuolar contents to the outside, through a pure which is located directly above each vacuole and is fixed in position.

Each radial canal or feeder consists of:

(i) A tubular terminal past

(ii) An ampulla and

(iii) An injector.

The fluid from the protoplasm surrounding the radial canal passes into the terminal part which is also able to secrete hypertonic fluid into the lumen. This fluid is collected in the ampulla which incomes bulb-or flask-shaped when distended (diastole of the ampulla), but when the fluid is passed out (systole of the ampulla) it is of the same diameter as the terminal portion.

In place of one flask-shaped enlargement sometimes the ampulla consists of a series of bulb-like swellings. When the ampulla is fully distended (diastole) it collapses and the fluid is passed through the injector to form the contractile vacuole. Thus it is apparent that the systole of the ampulla becomes the diastole of the vacuole. In some species of Paramecium (P. trichium), the contractile vacuole is without radial canals.

Food Vacuoles:

These are roughly spherical, non- contractile bodies varying in size and number lying in the endoplasm. They contain ingested food particles, principally bacteria and a small amount of fluid bounded by a thin definite membrane. Volkonsky (1934) proposed the name gastrioles for these vacuoles. Associated with the food vacuoles are the digestive granules.

Kappa Particles:

There are certain unusual endoplasmic components in some species (P. nurelia). These are in the form of very minute particles capable of self-reproduction, containing nucleic acids and appear to be bacteria. Previously these were collectively known as Kappa particles, although they are now recognized as several distinct kinds such as kappa, pi, mu, lambda, etc. They are concerned with cytoplasmic inheritance.

They are infective to individuals which lack them and for this reason they are now regarded as symbionts. Kappa particles are rod-like. Paramecia having kappa particles are designated ‘killers’ and those without, ‘sensitive’. The ‘killers’ produce a toxic- substance, called paramecin, which kills the ‘sensitives’.

Thus, the killers have a slow lethal effect on the sensitives in the same culture. Kappa particles can survive only in the presence of a particular dominant nuclear gene, K. During conjugation, if a ‘sensitive’ paramecium lacking the dominant gene receives cytoplasm containing kappa particles, the latter will not survive.

However, if the ‘sensitive’ has the dominant gene, the kappa particles will survive and multiply, and the ‘sensitive’ also becomes a ‘killer’. If a ‘killer’ paramecium loses the dominant gene at autogamy, its kappa particles disappear and it becomes ‘sensitive’.

Pi, which is a mutant of kappa neither protects the paramecia having it, nor kills those without it. Mu is a killer and destroys its mate during conjugation. Lambda induces lysis in certain sensitive paramecia.