Example of Phylum Protozoa: Paramoecium

In this article we will discuss about Paramoecium:- 1. Habit, Habitat and Structure of Paramoecium 2. Locomotion of Paramoecium 3. Nutrition 4. Respiration 5. Osmoregulation and Excretion 6. Reproduction.

Habit, Habitat and Structure of Paramoecium:

Paramoecium is found in stagnant water of ponds, ditches, pools, reservoirs, rivers, streams rich in organic matter. They are free-living and omnivorous in habit. Paramoecium is cultured in the laboratory in hay infusions. The type described here is known Paramoecium caudatum.

Structure:

Size and shape:

Paramoecium can be seen with naked eyes as white specks moving about very rapidly in the medium. It meas­ures about 0.25 mm in length.

Paramoecium looks like a slipper and hence is called slipper animalcule. Body is some­what cylindrical but flattened (Fig. 10.36). One end of the body is slender and blunt and remains foremost during locomotion. Obvi­ously this is the anterior end.

The other end or the posterior end is thick and pointed. The surface of the body which is with a groove and always faces the substratum is called the oral or ventral surface while the opposite is aboral or dorsal surface.

Pellicle:

Body is covered by a thin, dis­tinct, tough, flexible membrane called the pellicle. It gives a definite shape to the or­ganism. The pellicle is sculptured into a large number of polygonal or hexagonal depres­sions with their raised margins (Fig. 10.37A). The depressions are provided with holes at the centre through which the cilia project.

Ultrastructure of the pellicle:

Ehret and Powers (1959), Pitelka (1965) have presented the electron microscopic study of the pellicle (Fig. 10.37A). The polygonal or hexagonal areas correspond to regular series of closely packed elongated, flattened vesi­cles, the alveoli, from which cilia arise. The anterior and posterior margins of the polygo­nal areas bear the openings of trichocysts.

There are three membranes in the pellicle of which there is an outer limiting plasma membrane which is continuous with the membrane surrounding the cilia. The inner membranes lying beneath the outer mem­brane enveloping the body which form a series of closely packed, flattend vesicles, called alveoli. The outer and inner mem­branes of the alveoli thus form a middle and inner membrane of the pellicle.

Cilia:

The cilium is a short, stiff thread­like locomotory organelle and develops from the body surface. It originates from a basal body or kinetosome.

The whole body is covered by cilia and the condition is called holotrichous. The number of cilia varies from 10-14 thousands. The cilia are arranged in longitudinal rows and all are of equal size except a few at the posterior extremity which are longer and constitute the caudal tuft.

A cilium is about 10—12 µ long and 0.27 µ in breadth.

It consists of two parts:

(i) a basal body or kinetosome which lies embedded in the ectoplasm and

(ii) a shaft-short thread­like structure lies above the pellicle.

The basal body is a compact, spherical body and ho­mologous with the centriole.

Ultra structure of cilia:

Shaft:

The shaft of the cilium (Fig. 10.38A) is composed of a bundle of microtubules or sometimes called fibrils, called the axoneme surrounded by a cytoplasmic sheath which is continuous with the pellicle (Fig. 10.38A).

The distal end of the cilium tapers to a point where the number of fibrils is reduced (Fig. 10.38A). In cross section (Fig. 10.38B) the axoneme is composed of a ring-like 9 double peripheral microtubules, situated around two central unpaired microtubules (9/9 + 2).

Each peripheral filament or fibril is composed of two microtubules or sub-fibres, hence called doublet and two unpaired central micro­tubules are called singlets. The diameter of the axoneme is 0.21 µm.

The central singlet microtubules are surrounded by a central sheath and these two microtubules form the central shaft of the cilium (Fig. 10.64B). The space between central sheath and peripheral tubules is called matrix region. The 9 periph­eral fibrils are radially arranged at a distance of 200 Å from each other.

All the microtubules of the axoneme are composed of a globular protein, called tubulin. The microtubules are hollow cylinder-like structure and the inner tubule of each doublet is called A tubule and the outer tubule is called B tubule, of which A is small and com­plete and B is comparatively larger and incomplete. The tubules are microtubules.

A sub fibril or subfibre (microtubule) contains 13 protofilaments and B has 10 to 11. The protofilaments are sometimes called tubulin subunits. Nine radially arranged spokes originate from each A tubule of the peripheral doublets and extend towards the central sheath. A pair of arms called dynein arms which project from A tubule arranged in a clockwise direction towards the B tubule of the neighbouring doublet.

The dynein arms are composed of a protein called dynein. The peripheral doublets are connected by a bridge, called nexin links, composed of a protein nexin. The central two singlet microtubules are called namely C₁ and C₂ and dynein arms and radial spokes are ab­sent (Fig. 10.64B).

Basal body:

A cross section of the basal bodies shows a ring of nine triplet peripheral groups of microtubules or may be des­ignated as A, B and C tubules in each group.

The innermost A tubule is circular in outline and is connected to a crescent-shaped B and C tubules. The tubule is designated as mi­crotubule. A central cylinder-like structure without any singlet, called the hub. The dynein arms are absent in any tubule of the triplets (Fig. 10.64C).

All the fibrils of the axoneme in cilia or flagella are anchored into a plate-like struc­ture called basal plate. The basal plate is connected by two dense cytoplasmic proc­esses, called basal bodies. From each basal body arises deeply rooted hair-like proteinaceous structures called rootlets (Fig. 10.38A).

The function of the nexin links is un­known but may act as stimulators which help to maintain the integrity of the axoneme during the sliding motion. The protein dynein contains a high molecular weight ATPase which cleaves ATP releasing energy, causes a power stroke. The basic mechanism of cili­ary motion is related to the interaction be­tween dynein and tubulin.

Internal Structure:

Cytoplasm:

Beneath the pellicle the pro­toplasm is differentiated into cortex and medulla. The cortex or ectoplasm is clear, non-granular, less extensive and contains spindle-shaped trichocysts. The medulla or endoplasm is granular, semifluid and bears nuclear apparatus, contractile vaculoles, food vacuoles, various organelles (golgi bodies, mitochondria, ribosomes), etc.

The ectoplasm bears the following sys­tem:

1. Infraciliary system:

The infraciliary system is constituted by the basal bodies or kinetosomes, located in the alveolar layer of ectoplasm, and kinetodesmata.

(i) Basal bodies or kinetosomes:

The basal body or kinetosome is a spherical body lying embedded in the ectoplasm and from which each cilium originates.

(ii) Kinetodesmata:

A group of fibrils that develop from each row of basal bod­ies, running along the right side, to­gether called the kinetodesmata. The individual fibrils do not run anteriorly farther than the five basal bodies. The cilia, kinetosomes and kinetodesmas together form a unit, called kinety.

All the kineties are associated to form infraciliary system. It was supposed that the infraciliary system co-ordi­nates the beating of cilia but the role of infraciliary system has not yet been conclusively demonstrated (Naitoh and Eckert, 1969). Infraciliature is also a tool by which the taxonomists use to distinguish the different ciliate species and the degree of differences on which the relationship is established.

(iii) Neuromotor system:

Lund (1933) observed under light microscope and reported that some other types of fibrils remain connected to the kinetosomes or basal bodies called neuronemes or myonemes. These are highly contractile and play the role in the movement of cilia.

Another a small bilobed mass is situated on the wall of cytopharynx, which is formed by the converge of neuronemes, called the motorium. The kinetosomes, neuronemes and moto­rium constitute the neuromotor sys­tem. But electron microscopic studies do not reveal such organ.

2. Trichocysts (Fig. 10.37A, B):

Trichocysts are spindle-shaped or bottle shaped bodies embedded in the ectoplasm alternating with the alveoli. They lie their long axes perpen­dicular to the body surface in between the basal granules. The length of trichocyst is about 8 µm and the breadth is 2 µm. It con­sists of an oval-shaped shaft and a spine-like structure at the outer end, called spike, which is covered by a cap.

The shaft is not found at the un-discharged state and probably poly­merised in the process of discharge. The trichocysts are filled with homogenous, re­fractive and semi-fluid substances with a fibrous protein. The fibrous protein is trichinin and calmodulin.

On being stimu­lated the content comes out through small openings on the ridges of the pellicle. When initiated by external stimulus, the trichocysts themselves are expelled out of the body as a long, striated thread-like shaft with a barb on the top (Fig. 10.37B), seen under electron microscope.

The function of trichocysts is not known clearly. It is believed that these organelles act as defensive organ to predators. Other func­tion may be that the trichocysts secret some substance which acts as adhesive and helps to anchor the animal to the substratum.

Nuclear apparatus:

The endoplasm bears at the centre the nuclear apparatus con­sisting of a large kidney-shaped mega-or macronucleus, and a small rounded micronucleus possesses an inconspicuous nuclear membrane and many nucleoli. The macronucleus is also called somatic or vegetative or trophic nucleus and controls the meta­bolic activities of the cell. It is also concerned with the synthesis of RNA and DNA.

The micronucleus is the reproductive nucleus which possesses a definite nuclear membrane and a definite number of chromosomes. It is concerned with the synthesis of DNA and controls the reproductive activities of the in­dividual. Of the two amino acids (DNA and RNA), RNA is said to be present a large amount than DNA in both nuclei.

In P. caudatum there are two nuclei— one mega and the another micronuclei. But the number varies in different species. In P. aurelia, there are a single macronucleus and two micronuclei; in P. multimicronucleatum, there are a single macronucleus and four or more micronuclei and in P. polycaryum there are three to eight micronuclei and a single meganucleus.

Contractile vacuoles:

There are two star- shaped, liquid filled contractile vacuoles situated one on either end of the body, close to the dorsal surface (Fig. 10.36). Each vacu­ole consists of a large central vacuole opens to the outside on the dorsal side through a discharge canal. Each contractile canal is surrounded by six to ten elongated canals, called radial canals, also called feeding ca­nals.

Each radial canal is divided into 3 parts:

(i) A proximal injector canal which opens into the contractile vacuole;

(ii) A mid­dle ampulla which is an inflated part and

(iii) A terminal part which is extended into the endoplasm and a network of minute tubules, called nephridial tubules, are asso­ciated with the terminal part (Fig. 10.39).

The function of contractile vacuoles is osmoregu­lation and excretion. Day (1930) reported that contractile vacuole is a hydrostatic organ which also helps to eliminate the metabolic wastes.

Food vacuoles:

Food vacuoles are many and are found in the endoplasm. They con­tain inside them food particles at different stages of digestion (Fig. 10.36).

Oral apparatus:

On the ventral side of the body is situated a broad shallow groove called the oral groove (Fig. 10.36). The oral groove runs obliquely backward into a fun­nel-like structure called vestibule. The ves­tibule leads into a tubular passage called the buccal cavity. The buccal cavity leads into an aperture called cytostome or mouth (Fig. 10.37C).

The cytostome opens into a wide funnel-shaped depression called gullet or cytopharynx (Fig. 10.37C). A definite open­ing is situated posterior to the mouth (cytostome), through which undigested food particles are eliminated, called cytopyge or anal spot (or called cytoproct) (Fig. 10.36).

In the oral passage the cilia show a vari­ation in size and form. The cilia in the buccal cavity are fused in a crescentic manner to form an endoral membrane. It runs transversely along the right wall and marks the junction of the vestibule and buccal cav­ity.

The type of ciliary organelle found in the buccal cavity which is formed by two or three rows of short cilia, all of which adhere to form a triangular or fan-shaped plate, called membranelle.

A special ciliary structure is formed by the fusion of four rows of cilia, called peniculus (Fig. 10.37D) which is a modified membranelle and greatly length­ened, and similar to an undulating mem­brane in function. Two peniculi such as dor­sal peniculus and ventral peniculus are found in the buccal region.

Locomotion of Paramoecium:  

Locomotion is the move­ment of animals from one place to another. The locomotory organelles of Paramoecium are cilia.

Paramoecium exhibits two types of movement:

(i) Creeping movement or Metaboly,

(ii) Swimming or ciliary movement.

Creeping movement:

During creeping movement the animal uses its cilia of the oral surface as miniature legs and simply glides over the obstacles. As the pellicle is thin and elastic, the animal can easily bend and squeeze through gaps narrower than its own body diameter.

Swimming or ciliary movement:

The animal can swim forwards and backwards. The swimming is effected by the beating of cilia. The cilia may be compared with the oars of rowing boat. The cilia of a progress­ing animal bend throughout its length and strike the water, and thus the cilia usually bend backward and their beating drives the animal forward. The backward movement of cilia is called effective stroke.

The beating of the cilia can be compared with the oscillation of the pendulum.

Each oscillation of cilia consists of two strokes:

(i) The effective stroke in which the shaft of the cilia becomes stiffened and slightly curved to strike the water like an oar. As a result of the effective stroke the animal drives forward and

(ii) Re­covery stroke in which there is no move­ment of the animal.

During the recovery stroke the cilium becomes loose and brings to its original position. It becomes ready for its next effective stroke. The cilia of the trans­verse row beat simultaneously and in longi­tudinal row the cilia beat sequentially i.e., one after another. The sequential activation of cilia shows like a wave, called metachronous rhythm.

The animal swims in an elongated spiral path and the individual body rotates upon its own longitudinal axis. This is possible due to the longitudinal arrangement of cilia on the body surface (Fig. 10.40).

There is evidence that the infraciliature and cytoplasm play a major role in the or­ganised movement of cilia. The direction and intesity of the beat of cilia are controlled by changing levels of Ca⁺⁺ and K⁺ ions. The organised movement of cilia is also control­led by the neuronema or neuromotor sys­tem in the body.

Nutrition in Paramoecium:  

Paramoecium is a typically holozoic ani­mal because the animal engulfs or ingests the solid food materials.

Food:

Paramoecium feeds on bacteria, other protozoa, unicellular algae, yeast and minute particles of animal, so it is called microphagy. Microphagy means process of collection and ingestion of small food particles. Paramoecium is called omnivorous animal because it feeds on both plants and animals.

Feeding mechanism:

The cilia lining the oral groove perform a great role in capturing food particles. During ingestion these cilia make a co-ordinated beating and as a result a continuous current of water containing food particles passes down the gullet or cytopharynx. The wall of the gullet or cytopharynx is lined with micro-tubular rods, called nematodesmata, supports the walls of the pharynx and helps the inward transport of food vacuoles.

The food particles are be­lieved to be paralysed by the toxic product of the trichocysts. The food particles are collected at the bottom of the gullet and are worked into balls by the cilia. These food balls or bolus, along with some water, pass through the cytostome to form food vacuoles in the endoplasm.

Cyclosis:

The formation of food vacuole is a continuous process and as soon as one food vacuole detaches from the gullet an­other starts forming. The food vacuoles move in a definite course. The food vacuoles first travel to the posterior end and then take a turn and travel anteriorly.

They reach the anterior border of endoplasm and travel back and come to the middle of the body to com­plete the circulation. This cyclical movement of the food vacuoles in a definite course, by its streaming movement, is called cyclosis or circulation. The colour of the contents gradu­ally changes from green to yellow.

Digestion:

During cyclosis the food materials are killed, digested and absorbed and the mode of digestion is similar of that of Amoeba. With the formation of food vacu­ole, the acidic vesicles (acidosomes) are fused with the food vacuole. As a result the food vacuole becomes highly acidic (pH becomes 3) and the food materials are killed.

Now lysosomes are joined with the food vacuole. So the lysosomal contents are highly acidic and is not efficient for enzymatic activity. When the food vacuole moves forward, the pH rises from 3 to 5. Digestion occurs in between 4.5 to 5 in most ciliates. The animals can digest proteins and carbohydrates but not fats. The enzymes protease breaks the protein into amino acids and carbohydrate into glucose.

Absorption:

After digestion, the simpler forms of the food materials are absorbed in the endoplasm.

Egestion:

After digestion, undigested residue of food is thrown out through the cytopyge (or called cytoproct or cell anus) situated on the ventro-posterior surface. The cytopyge is only visible during the act of excrement.

Respiration in Paramoecium:  

Oxygen, dissolved in water, enters the body through the surface by diffusion and carbon dioxide comes out in the same man­ner.

Osmoregulation and Excretion in Paramoecium:

The two contractile vacuoles (Fig. 10.36) are effective osmoregulatory organelles. Each contractile vacuole is provided with 6-11 radiating or inhalent canals (Fig. 10.39A, B) which go deep into the endoplasm and collect excess of water.

The inhalent canals pour their contents into the central vacuole which after attaining maximum size bursts to liberate the contents outside the body. Concurrent contrac­tion of the two central vacuoles does not take place. The time lapse between the contraction of two vacuoles is usually 10-12 seconds.

Some workers consider the radiating ca­nals as formative vacuoles. They have put forward the view that after the contraction of the central vacuole, the formative vacuoles come together and fuse at their inner ends to form a new central vacuole round which new formative canals gradually develop.

Reproduction in Paramoecium:

Paramoecium reproduces both by asexual and sexual methods.

Asexual reproduction:

It takes place by transverse binary fission and is the common mode of reproduction (Fig. 10.41). During binary fission the micronucleus divides eumitotically, i.e., by passing through all the stages of mitosis and the macronucleus divides amitotically. The products of these divisions are two macronuclei and two micro- nuclei.

One macronucleus and one micronucleus go to the anterior part while the other pair go to the posterior part. Finally, a constriction appears in the middle of the body of the animal and two daughter paramoecia are formed. The oral groove is usually inher­ited by the daughter at the anterior end. How­ever, in both of them the division is always followed by regeneration of lost parts.

Duration:

The whole process of division normally takes about 2 hours. The rate of division is dependent on availability of food and on tem­perature. Two to three divisions are not uncommon in 24 hours-time.

Sexual reproduction:

After practising binary fission for a considerable number of generations, the paramoecia make a nuclear reorganisation by conjugation or mating (Fig. 10.42).

Conjugation:

Definition:

Conjugation is a temporary union of two individuals of the same species but two different mating types for the purpose of mutual exchange of nuclear material through the formation of a cytoplasmic channel.

Conjugation occurs among ciliates and suctorians.

The individuals participating in conjuga­tion are called conjugants. Individuals which are usually fertile and belong to a same spe­cies, but can be differentiated on the basis of mating behaviour, are called varieties or syngens. Sonneborn reported a number of varieties within each species of Paramoecium.

The individuals which are morphologically alike but physiologically and genetically apart, called mating types. Mating types are designated in Roman letters and are written as mating type I, mating type II, etc. The same members of the mating type I never participate in conjugation. Conjugation takes place between the members of the different mating types, such as members of the mat­ing type I and mating type II or III.

Discovery of the process:

The first account of conjugation in P. caudatum has been given by Butschli, Maupas (1889). In P. aurelia, classical account of conjugation is provided by Hertwig and Maupas in 1889. The basic pattern is similar but in detail it is slightly different in different species of paramoecium.

Factors for conjugation:

The food, tem­perature, light, bacterial strength are related to induce the conjugation of different species of Paramoecium.

(i) Conjugation does not occur in ex­tremely starved condition or in well fed condition. It is seen only when the shortage of food takes place in the medium.

(ii) Sudden darkness in the lighted con­dition and low temperature play a role in inducing the conjugation process and does not take place in between 19-37°C temperature range.

(iii) Conjugation occurs throughout day and night except the late night and at dawn. Light plays a role to induce the conjugation in different varieties of P. aurelia.

(iv) In P. caudatum, conjugation does not proceed further after the exchange of nuclear material if it induces in cul­ture media.

(v) Certain agents like Mg, K and heparin are said to induce the conjugation in culture media under Ca poor condi­tion, and the substance such as acetamide increases the rate of conjugation.

(vi) Under Ca poor condition, certain chemical agents like Mg and K can liquify the ectoplasmic part of the cytoplasm partially and help the ani­mal to become sticky along the ante­rior surface and brings the animals together on the oral surface.

(vii) A protein substance, called immaturin, is stored in the cytoplasm of the mat­ing type individuals that induces con­jugation. This protein is formed only if the daughter individuals take a rest after conjuga­tion.

Process:

Following steps have been re­corded in conjugation of P. caudatum (Fig. 10.42). The conjugants come close together and pair by the ventral surface. The inter­locking between them is made stronger by the gullets which degenerate to form a pro­toplasmic bridge between them.

(I) The macronucleus undergoes gradual disintegration and ultimately disappears.

(II) The micronucleus which is diploid, undergoes two successive divisions forming four haploid micronuclei (sometimes called pronuclei) in each of the conjugants. One of the divisions is probably meiotic in nature.

(III) Three of these four micronuclei in each conjugant degenerate and the remain­ing one undergoes mitosis to form two ga­metic nuclei. One of the gametic nuclei is large and is called the stationary pronucleus while the small one is called the migratory pronucleus.

(IV) The migratory nucleus of one conju­gant goes to the stationary nucleus of the other and vice versa through the protoplas­mic bridge.

(V) The migratory pronucleus of one con­jugant ultimately unites with the stationary pronucleus of the other and forms the zygote nucleus or synkaryon which restores the diploid condition.

(VI) The conjugants with the zygote nucleus now separate after the union of about 12 to 48 hours, and are called exconjugants.

(VII) In each exconjugant the zygote nu­cleus undergoes three successive mitotic di­visions forming eight nuclei.

(VIII) Of these eight nuclei four enlarged become macronuclei and four smaller become micronuclei. Later on three of the four micro- nuclei degenerate leaving behind one active micronucleus.

(IX) The micronucleus divides mitotically and cytoplasmic division follows resulting into two paramoecia from each exconjugant and each of the two paramoecia is provided with two macronuclei and one micronucleus.

(X) The micronucleus divides again, fol­lowed by the cytoplasmic division, resulting four paramoecia from each exconjugant, each with one micro and one macronucleus.

(XI) Thus from each exconjugant four paramoecia are formed.

Remarks:

The observation of the electron microscopic studies in P. caudatum reveals that the exchange of nu­clear material takes place through the small cytoplasmic channels which are about 0.25 µ in diam­eter and broad cytoplasmic bridge is not formed.

Duration:

From the beginning of the con­jugation of Paramoecium the final division occurs after about 18 hours. The duration varies in different species.

Changes during Conjugation:

Morphological:

(i) Conjugants are smaller is size than the non-conjugating individuals and are slower in movement.

(ii) Food vacuoles are diminished just before conjugation.

(iii) Oral surface becomes sticky.

Physiological:

(i) More O₂ consumption takes place.

(ii) Glycogen content decreases.

Significance:

After a number of repeated binary fissions, the daughter Paramoecia become weak and ill, so to gain vitality and physiological efficiency conjugation takes place among paramoecia. Hyman (1940) regarded the process as “the sexual phenom­enon” but others consider it to be an episode in reproduction.

It helps in the following cases:

(i) After a long repeated binary fissions, the daughter individuals become weak and die. So conjugation helps in re-juvenescence to gain vigour and vitality and avoid the senile decay of the race.

Maupas, Calkin and Hertwig sug­gested that conjugation makes the conjugants active and Maupas also suggested that conjugants prolong their maturity after conjugation. Oth­ers claim that after conjugation they become inactive and old, but if they get any change for conjugation they again gain the vitality.

(ii) Due to exchange of nuclear material, the macronucleus and micronucleus are totally reorganised and become a new one. Due to the reorganisation the macronucleus controls the meta­bolic activities of the animal, and the vitality and vigour of the individual are enhanced.

(iii) It produces new type of genetic resuffling. Due to genetic variation, the different mating types appear among population.

(iv) The nuclear apparatus is reorganised and readjustment takes place between the two individuals.

Autogamy:

This is a sort of regularly recurring nuclear reorganisation in solitary Paramoecium aurelia. Some of these nuclear changes are almost identical to the changes which occur during conjugation. The chief difference is that fusion occurs between two nuclei which originate in a single paramoecium. The phenomenon has been termed autogamy (Fig. 10.43). In Paramoecium aurelia there are two micronuclei and one macronucleus.

During the process the two micronuclei divide twice forming eight mi­cronuclei. Seven of these micronuclei degen­erate and the remaining one divides for the third time producing two functional nuclei. The functional nuclei travel to the paraoral cone and fuse to form the synkaryon. Paraoral cone forms as a projection at the level of oral groove.

The synkaryon divides twice to form four nuclei. Two of these four nuclei now metamorphose into macronuclei. The old macronucleus in the mean-time undergoes fragmentation and disintegration. By another division of the cell and micronucleus two paramoecia are formed having normal nu­clear apparatus.

In conjugation, possibilities are there for variation whereas autogamy gives pure lines. It has been observed that autogamy occurs after the emergence of a depression stage in the individual. This depression stage is always indicated by an increase of volume and definite sluggishness on the part of the animal.

Irritability:

(I) Response to mechanical stimuli:

In Paramoecium the anterior end is more sensi­tive than the posterior end and offers posi­tive or negative responses.

(II) Response to gravity:

Paramoecium exhibits negative reaction to gravity and in a culture jar resides just below the surface of the medium.

(III) Response to chemical stimuli:

Paramoecium shows positive reaction to weak chemical solutions of acids and negative reac­tion above certain concentration. Paramoecium can thrive in a drop of medium containing 0.02 per cent acetic acid but if the concentra­tion of the acid is increased the animal moves away and takes refuge in the periphery where the acid is diluted by the surrounding water.

(IV) Response to electrical stimuli:

Paramoecium moves to the anode when elec­tric current is passed through the medium where it lives.