''Paramecium caudatum'' is a species of

unicellular A unicellular organism, also known as a single-celled organism, is an organism that consists of a single cell, unlike a multicellular organism that consists of multiple cells. Organisms fall into two general categories: prokaryotic organisms and ...

protist A protist ( ) or protoctist is any eukaryotic organism that is not an animal, land plant, or fungus. Protists do not form a natural group, or clade, but are a paraphyletic grouping of all descendants of the last eukaryotic common ancest ...

in the

phylum In biology, a phylum (; : phyla) is a level of classification, or taxonomic rank, that is below Kingdom (biology), kingdom and above Class (biology), class. Traditionally, in botany the term division (taxonomy), division has been used instead ...

Ciliophora The ciliates are a group of alveolates characterized by the presence of hair-like organelles called cilia, which are identical in structure to eukaryotic flagella, but are in general shorter and present in much larger numbers, with a different ...

.101 Science.com: Paramecium Caudatum
/ref> They can reach 0.33 mm in length and are covered with minute hair-like

organelle In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...

s called

cilia The cilium (: cilia; ; in Medieval Latin and in anatomy, ''cilium'') is a short hair-like membrane protrusion from many types of eukaryotic cell. (Cilia are absent in bacteria and archaea.) The cilium has the shape of a slender threadlike proj ...

.Paramacium
/ref> The cilia are used in locomotion and feeding. The species is very common, and widespread in marine, brackish and freshwater environments.Carey, Philip G. Marine interstitial ciliates: an illustrated key. 1992. p. 128

Appearance and physical characteristics

''Paramecium caudatum'' is 170–330 micrometres long (usually 200–300 micrometres). The cell body is spindle-shaped, rounded at the front, tapering at the posterior to a blunt point. Early microscopists likened its shape to that of a slipper, and commonly referred to it as the "slipper animalcule." The pellicle is uniformly covered with cilia, and has a long oral groove, leading to deeply embedded oral cavity, lined with cilia (short, hair-like protoplasmic processes that serve as organs of locomotion and food capture). ''P. caudatum'' has two

contractile vacuole A contractile vacuole (CV) is a sub-cellular structure (organelle) involved in osmoregulation. It is found predominantly in protists, including unicellular algae. It was previously known as pulsatile or pulsating vacuole. Overview The contrac ...

s, which serve to excrete excess water taken up from the outside, regulating the water contents of the body. Radially distributed "collecting canals" give the contractile vacuoles a distinctive star-like shape. The cell is enclosed by a cellular envelope (

cortex Cortex or cortical may refer to: Biology * Cortex (anatomy), the outermost layer of an organ ** Cerebral cortex, the outer layer of the vertebrate cerebrum, part of which is the ''forebrain'' *** Motor cortex, the regions of the cerebral cortex i ...

) densely studded with spindle-shaped

extrusome Extrusomes are membrane-bound organelles found in eukaryotic cells that are capable of discharging material contained within to the exterior of the cell. Due to the diversity in structure and function, it is unlikely that different types of extruso ...

s called

trichocyst A trichocyst is an organelle found in certain ciliates and dinoflagellate The Dinoflagellates (), also called Dinophytes, are a monophyletic group of single-celled eukaryotes constituting the phylum Dinoflagellata and are usually considered pr ...

Feeding and movement

''P. caudatum'' feed on

bacteria Bacteria (; : bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of Prokaryote, prokaryotic microorganisms. Typically a few micr ...

and small

eukaryotic The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...

cells, such as

yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom (biology), kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are est ...

and

flagellate A flagellate is a cell or organism with one or more whip-like appendages called flagella. The word ''flagellate'' also describes a particular construction (or level of organization) characteristic of many prokaryotes and eukaryotes and the ...

algae Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...

. the accumulated food particles, at the posterior end of the cytopharynx, are directed by the long cilia into the rounded, ball-like mass in the endoplasm called food vacuole. The food vacuoles are circulated by the streaming movement of the endoplasm which is called cyclosis. In

hypotonic In chemical biology, tonicity is a measure of the effective osmotic pressure gradient; the water potential of two solutions separated by a partially-permeable cell membrane. Tonicity depends on the relative concentration of selective memb ...

conditions (freshwater), the cell absorbs water by

osmosis Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively permeable membrane, selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of ...

. It regulates osmotic pressure with the help of bladder-like contractile vacuoles, gathering internal water through its star-shaped radial canals and expelling the excess through the

plasma membrane The cell membrane (also known as the plasma membrane or cytoplasmic membrane, and historically referred to as the plasmalemma) is a biological membrane that separates and protects the interior of a cell from the outside environment (the extr ...

. When moving through the water, they follow a spiral path while rotating on the long axis.

Reproduction and life cycle

''Paramecium caudatum'' has two nuclei (a large macronucleus and a single compact micronucleus). As in other ciliates, individuals cannot survive without the macronucleus and cannot reproduce without the micronucleus. ''Paramecia'' reproduce asexually, by

binary fission Binary may refer to: Science and technology Mathematics * Binary number, a representation of numbers using only two values (0 and 1) for each digit * Binary function, a function that takes two arguments * Binary operation, a mathematical o ...

. During reproduction, the macronucleus splits by a type of amitosis, and the micronuclei undergo

mitosis Mitosis () is a part of the cell cycle in eukaryote, eukaryotic cells in which replicated chromosomes are separated into two new Cell nucleus, nuclei. Cell division by mitosis is an equational division which gives rise to genetically identic ...

. The cell then divides transversally, and each new cell obtains a copy of the micronucleus and the macronucleus. Fission may occur as part of the normal vegetative

cell cycle The cell cycle, or cell-division cycle, is the sequential series of events that take place in a cell (biology), cell that causes it to divide into two daughter cells. These events include the growth of the cell, duplication of its DNA (DNA re ...

. Under certain conditions, it may be preceded by self-fertilization (autogamy), or it may follow

conjugation Conjugation or conjugate may refer to: Linguistics *Grammatical conjugation, the modification of a verb from its basic form *Emotive conjugation or Russell's conjugation, the use of loaded language Mathematics *Complex conjugation, the change o ...

, a sexual phenomenon in which ''Paramecia'' of compatible mating types fuse temporarily and exchange genetic material. During conjugation, the micronuclei of each conjugant divide by

meiosis Meiosis () is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, the sperm or egg cells. It involves two rounds of division that ultimately result in four cells, each with only one c ...

and the

haploid Ploidy () is the number of complete sets of chromosomes in a cell (biology), cell, and hence the number of possible alleles for Autosome, autosomal and Pseudoautosomal region, pseudoautosomal genes. Here ''sets of chromosomes'' refers to the num ...

gametes pass from one cell to the other. The gametes of each organism then fuse to form

diploid Ploidy () is the number of complete sets of chromosomes in a cell, and hence the number of possible alleles for autosomal and pseudoautosomal genes. Here ''sets of chromosomes'' refers to the number of maternal and paternal chromosome copies, ...

micronuclei. The old macronuclei are destroyed, and new ones are developed from the new micronuclei.Prescott, D. M., et al. "DNA of ciliated protozoa." Chromosome 34.4 (1971): 355–366. Without the rejuvenating effects of autogamy or conjugation a ''Paramecium'' ages and dies. Only opposite mating types, or genetically compatible organisms, can unite in conjugation.

References

{{Authority control Oligohymenophorea Protists described in 1834 Taxa named by Christian Gottfried Ehrenberg Ciliate species