In this article we will discuss about:- 1. Habitat and Structure of Paramoecium 2. Nutrition Required by Paramoecium 3. Respiration and Excretion 4. Locomotion 5. Behaviour 6. Reproduction.
Habitat and Structure of Paramoecium:
Paramoecium, known popularly as the slipper-animalcule, is a very small creature about 0.3 mm. in length and just visible to the naked eye. The animal is abundantly found in stagnant water containing decaying vegetable matter, feeding voraciously on the bacteria that swarm in such a polluted environment. Microscopic examination is necessary to reveal details of its structure.
Paramoecia are elongated and slipper-shaped. The anterior end of the animal is bluntly rounded but the posterior end is pointed. Thus the anterior end resembles the heel and the posterior end the toe of a slipper.
On the ventral surface, near the anterior end, is a depression leading into the oral groove or peristome which runs obliquely and becomes continuous through a small mouth, the cytostome, with a narrow tubular gullet or cytopharynx.
The gullet ends blindly inside the endoplasm. The whole surface of the animal is covered with minute, hair-like, protoplasmic projections called cilia which arise from granular structures called basal bodies.
The animal is composed of soft jelly-like protoplasm which is divisible into an outer, clear ectoplasm and an inner granular endoplasm. The ectoplasm is covered by a distinct pellicle. The pellicle is divided into a number of roughly hexagonal pits, from the centre of each of which arises a cilium.
Embedded in the ectoplasm are a large number of spindle-shaped bodies called trichocysts (thrix=hair). When irritated the trichocysts are discharged through pores which are situated on the opposing walls of the hexagonal areas already referred to. When discharged they form an effective jellylike barrier against an attacking enemy.
Moreover, the jelly-like substance holds the paramoecium to the substratum when the animal is feeding, otherwise it rotates like a rifle bullet and fails to collect its food. They are also used to entangle minute bacteria and other bodies upon which the slipper animalcule feeds. Embedded in the endoplasm near the centre of the animal, there is a large bean- shaped macronucleus.
A very much smaller micronucleus is closely applied to and lodged in a depression on the concave surface of the macronucleus. The macronucleus governs all the vital activities of the animalcule and the micronucleus plays a very important part during its reproduction. In P. aurelia, there are two micronuclei and one macronucleus.
Lying within the endoplasm there are numerous spherical food vacuoles for digesting food and two contractile vacuoles for eliminating waste products. Granular bodies are scattered through, out the endoplasm.
Some of these are protoplasmic granules; others are granules of reserve food materials and still others are excretory or secretory products. There is a weak spot posterior to the mouth, the temporary anal pore or cytopyge, for egesting undigested residue.
Nutrition Required by Paramoecium:
The food of Paramoecium consists of bacteria, yeasts, algae, and small protozoa. The cilia of the oral groove are more or less fused to form an undulating membrane which sweeps the food particles down through the cytostome into the cytopharynx.
At the posterior end of the cytopharynx the particles are surrounded by a watery vacuole. This, when fully loaded, constricts off, and begins to circulate in the endoplasm as a food vacuole by the rotary streaming movement called cyclosis. The food is digested by enzymes which are secreted into the food vacuole. The contents of the vacuole are at first acid, but the reaction soon becomes alkaline.
Digested food is absorbed and assimilated. With this, the food vacuole gradually diminishes in size. Meanwhile, by following the rotary streaming movement of the endoplasm (cyclosis), the food vacuole has covered a definite path through the body of the paramoecium.
At first it moves backward, then anteriorly, and then again backwards towards the weak spot or temporary anal pore. The indigestible residue in the food vacuole is finally egested through the anal pore. The path of cyclosis is indicated by arrowheads in Fig. 53 A.
Thus the mode of nutrition is holozoic.
Respiration and Excretion in Paramoecium:
There are two prominent contractile vacuoles, one at either end of the animal. Each consists of a rounded central space surrounded by and connected with a series of six to eleven narrow radiating channels which serve as feeders to the main vacuole. Nitrogenous waste products, excess of water, and carbon dioxide formed during respiration slowly collect in the vacuoles through the feeding channels.
The vacuoles increase in size and at the limit of dilatation they contract and burst, discharging their contents to the exterior. They are again slowly formed at their original site and the whole process is repeated, hence the name pulsating vacuoles.
These vacuoles are, therefore, excretory and respiratory in function. Oxygen required for respiration is absorbed through the pellicle from the dissolved air in the surrounding water.
Locomotion of Paramoecium:
Locomotion is rapid. It is effected by cilia which can beat either forwards or backwards. The animal, therefore, can swim in both directions. In normal circumstances, however, it runs forward. As the cilia beat obliquely backwards, the animal rotates on its long axis during its forward progression. The cilia in the oral groove are longer and more or less fused.
They beat obliquely against the axis of the groove and hence the anterior end of the animalcule is swerved to one side or the other alternately. As a result of the combination of forward motion, rotation, and lateral swerving, the animal is compelled to follow a spiral path. It can never swim in a straight line.
The ciliary motion, when viewed under the microscope, presents an undulating appearance comparable to the waves passing over a paddy field during a strong breeze.
Behaviour of Paramoecium:
Paramoecium, when it comes in contact with a solid object, recedes back and changing its course runs again till it can avoid the obstacle in its forward path. This is effected by a series of trial and error experiments. It is always kept informed about its environment by taking a sample of water in its oral groove and ejecting out the same. Any change of condition is thereby constantly and carefully noted.
The animal avoids injurious environment and by repeated efforts brings itself to a suitable position. Even food is rejected when there is no hunger. When hungry it roams about aimlessly in search of food. When violently stimulated by chemicals or an injurious enemy, the Paramoecium responds by throwing out trichocysts, which forms an effective barrier round the animal and protects it.
Reproduction in Paramoecium:
Paramoecium usually reproduces asexually by dividing into two daughter animalcules in a plane at right angles to the long axis of its body. This simple process is known as binary fission and it always occurs in a transverse plane. The division of the body is preceded by division of both the nuclei. The micronucleus divides mitotically into two exactly equal halves, but the macronucleus divides directly by amitosis.
The old gullet divides into two, so that each daughter is an exact but smaller replica of the original parent. The entire process is completed, in about two hours. If they are kept at a temperature of 15°C to 17°C. the daughter Paramoecia are ready to divide again at the end of twenty-four hours.
Raising the temperature to 20°C. results in two divisions during the course of a single day. When well-fed, Paramoecium may multiply asexually by fission for several months or even years.
Occasionally binary fission is interrupted by conjugation. This is a temporary union between two individuals which come together by opposing their ventral surfaces. A protoplasmic bridge is formed between them. The macronuclei of the two conjugants disappear and are absorbed into the cytoplasm.
The micronucleus of each conjugant divides twice to produce four nuclei—of which three degenerate. The remaining nucleus divides unequally into a larger stationary pro-nucleus and a smaller migratory pro-nucleus. There is mutual exchange of the migratory pro-nucleus between the two individuals and the migratory pro-nucleus of one fuses with the stationary pro-nucleus of the other.
The conjugants now separate and the single nucleus of each ex-conjugant divides thrice to produce eight nuclei, three of which are absorbed into the cytoplasm. Of the remaining five, four becomes macronuclei and undergo no further change whilst the other divides twice to produce four micronuclei.
In the meantime the ex-conjugant divides twice to produce four individuals, each with a macronucleus and a micro- nucleus. In all, therefore, eight new individuals are produced as a result of conjugation between two Paramoecia.
In all probability this occasional and peculiar sexual reproduction by conjugation is a natural provision to infuse new energy into an individual which has become old and tired by repeated binary fission. The process is, therefore, one of rejuvenation.
There is another process of rejuvenation in Paramoecium. It is known as endomixis. In this the macronucleus disappears. The micronucleus divides twice to produce four nuclei, of which three disintegrate. The remaining nucleus divides along with the division of the body—so that two Paramoecia, each with a single nucleus is formed. The single nucleus in each daughter Paramoecium divides twice to produce four nuclei.
Two of these are converted into macronuclei and the other two into micronuclei. The daughter Paramoecium now divides into two, each having one macronucleus and one micronucleus. Endomixis, then, produces four rejuvenated individuals from a single tired Paramoecium.
If condition of life becomes very hard and there is chance of impending death due to scarcity of food and water, the animal secretes a tough coat round it called cyst. In this encysted stage it tides over the difficult period in its life-history. With return of favourable condition, the cyst wall breaks and the animal comes out to begin a new life. Encystment, however, is very rare in Paramoecium.