Paramecium: Classification, Characteristics, and  Morphology

Paramecium: Classification, Characteristics, and  Morphology


Phylum: Protozoa

Class: Ciliata

Subclass: Euciliata

Order: Holotricha

Type: P. caudatum


Ciliates are distinguished by the existence of cilia on the body, two types of nuclei, one vegetative and the other reproductive, and conjugation, a unique form of sexual reproduction. About ten species of Paramecium exist, each with its shape, size, and structure. The majority of ciliates have complicated behaviour and advanced cellular organelles. Paramecium is found in freshwater, the sea, ponds, ditches, and streams, among other places, and is particularly prevalent in waterways containing decomposing organic materials. P. caudatum, P. aurelia, and P. bursaria are the most prevalent species.


1.P. caudatum is a microscopic ciliate that is between 180 and 300 micrometres in length.

2. It’s called a slipper animalcule because its body is elongated and resembles a slipper.

3. With a flattened ventral surface and a convex dorsal surface, the body is asymmetrical.

4. It has a rounded front end and a pointy back end.

5. The pellicle, a thin elastic cuticular membrane that maintains the paramecium’s distinctive shape, covers the body.

6. Cilia are hair-like protoplasmic structures that cover the whole body surface. A paramecium has between 10,000 and 14,000 cilia. Except for a few large ones in the back that create a caudal tuft, all cilia are of identical size, hence the specific name caudatum.

7. Each cilium is 10-12 mm long and is made up of an axial filament and a cytoplasmic sheath.

8. Each cilium is made up of 9 pairs of peripheral fibres and a pair of core fibres that are encased in a sheath under an electron microscope. The cilia perform a crucial function in movement and food digestion.


In P. caudatum the body’s cytoplasm is divided into two types: Exterior Ectoplasm and Inner Endoplasm.


It is the cytoplasm’s clean, narrow periphery region. Two significant structures can be found in the ectoplasm, which are:

A.Basal Granule or kinetosome

A tiny granule called a basal granule or kinetosome gives rise to each cilium. A fibril termed kinetodesmos grows from each basal granule and proceeds anteriorly, meeting up with other fibrils to form a bundle called kinetodesma. Only 5 fibrils make up each kinetodesma. The intraciliary system governs and coordinates the beating of cilia through the basal granules and related fibrils.

B. Trichocyst

These are spindle-shaped organelles that alternate with the basal granule in the ectoplasm. Each trichocyst has a tiny pore that connects it to the outside world. The trichocyst is made up of an elongated shaft and a spike at the end, which is covered by a cap. The shaft is filled with a thick fluid containing trichinine, a fibrous protein. The trichocysts are ejected to the exterior as an extended, fine thread on the body surface when the paramecium is aroused. The purpose of trichocyst is unknown. Some believe these are adhesion organelles, whereas others feel they are defence organelles.


Endoplasm is a granular, semi-fluid zone that encompasses the entire cell body. Each trichocyst has an extended shaft with a spike or barb at the end, which is protected by a cap. They’re most likely deployed as defence mechanisms. Macronuclei are huge, kidney-shaped polyploid organelles with a thin nuclear membrane. It has a vegetative role and regulates metabolic activity on a day-to-day basis. A spherical micronucleus with a nuclear membrane and a diploid number of chromosomes is present. It governs the animal’s reproductive processes and is stuck in a depression on the surface of the macronucleus.

It’s the cytoplasm’s interior, granular semifluid portion. It contains mitochondria, the Golgi body, and reserve food resources, among other things. Nuclear apparatus, contractile vacuoles, and feeding vacuoles are important endoplasm organelles.

Contractile vacuoles

In paramecium, there are two contractile vacuoles, one on each side of the body. Each vacuole is surrounded by 6-10 long and narrow radial canals that reach deep into the cytoplasm and link to the endoplasmic reticulum. The excretory system of the cell body is formed by contractile vacuoles. Within the endoplasm, food vacuoles known as gastrioles can be observed moving.

Food vacuole

In the endoplasm, numerous feeding vacuoles move with the cyclic movement of the endoplasm. The shape and size of food vacuoles are determined by the type of food. A large shallow oblique depression termed the oral groove runs from the middle to the anterior end of the paramecium’s ventral surface. There is an orifice termed the cystostome at the base of the oral groove, from which a funnel-like cytopharynx emerges. The cilia of the cytopharynx are organised differently. Three membranelles, two penniculi, and one quadrulus were formed as these cilia joined. In addition, a row of cilia fuses to form an endoral membrane right beyond the cytostome. All of this makes up the paramecium ingestory apparatus. A transient aperture called cytopyge is seen only during egestion near the posterior end of the body on the ventral side.

Nuclear apparatus

It is made up of a huge bean-shaped macro or maganucleus at the cytostome and a little circular micronucleus trapped in a meganucleus depression. The macronucleus is the vegetative nucleus, which governs the body’s metabolic activity, whereas the micronucleus manages the organism’s reproductive processes.

Respiration: In Paramecium, breathing occurs through the body’s general surface.

Osmoregulation: It is handled by contractile vacuoles. To get rid of surplus water from the cytoplasm, they constrict and expand at regular intervals. Water from the cytoplasm is discharged into the endoplasmic reticulum, from where it passes into the nephridial tubules and feeder canals, where it is stored in ampullae before being emptied into the contractile vacuole. The vacuole transfers extra water to the exterior through a pore in the pellicle once it has reached its maximum size.

External Morphology

The slipper-shaped P. caudatum has a diameter of 170-290 microns. The cell is extended, with a rounded front end and a somewhat pointed back end. The oral groove is a conspicuous, oblique dip on the body’s ventral surface that leads to a deeper conical vestibule that communicates with the buccal cavity. The pellicle, which is made up of a pattern of polygonal depressions and elevated lines with a series of cavities called alveoli, serves as the body’s external envelope. Each polygon has a single cilium protruding from its centre. Cilia, measuring 10-12 microns in length and 0.27 microns in diameter, cover the entire surface of the body.

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