Polyp
Encyclopedia
A polyp in zoology
is one of two forms found in the phylum
Cnidaria
, the other being the medusa
. Polyps are approximately cylindrical in shape and elongated at the axis of the body. In solitary polyps, the aboral
end is attached to the substrate
by means of a disc-like holdfast
called the pedal disc
, while in colonies
of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth
, and is surrounded by a circlet of tentacle
s.
Anthozoa
, comprising the sea anemones and coral
s, the individual is always a polyp; in the class Hydrozoa
, however, the individual may be either a polyp or a medusa, with most species undergoing a life cycle
with both a polyp stage and a medusa stage. In class Scyphozoa
, the medusa stage is dominant, and the polyp stage may or may not be present, depending on the family
. In those scyphozoans that have the larval planula
metamorphose
into a polyp, the polyp, also called a "scyphistoma," grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known as strobilation
. Once strobilation is complete, the polyp may die, or regenerate
itself to repeat the process again later. With Cubozoans, the planula settles onto a suitable surface, and develops into a polyp. The cubozoan polyp then eventually metamorphoses directly into a Medusa.
The body of the polyp may be roughly compared in a structure to a sac, the wall of which is composed of two layers of cell
s. The outer layer is known technically as the ectoderm
, the inner layer as the endoderm
(or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell layers of the body wall. The mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal
elements formed by cells which have migrated
into it from the ectoderm.
The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle of tentacle
s which resemble glove fingers. The tentacles are organ
s which serve both for the tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, containing coiled like stinging nettle-cells or nematocysts which pierce and poison and firmly hold living prey paralysing or killing them which include copepods and fish larvae. Longitudinal muscular fibrils formed from the cells of the ectoderm allow tentacles to contract when conveying the food to the mouth. Similarly, circularly disposed muscular fibrils formed from the endoderm permit tentacles to be protract or thrust out once they are contracted. These muscle fibres belonging to the same two systems, thus allows the whole body to retract or protrude outwards.
We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome
, in the centre of which again is the mouth. As a rule there is no other opening to the body except the mouth, but in some cases excretory pore
s are known to occur in the foot, and pores may occur at the tips of the tentacles. Thus it is seen that a polyp is an animal of very simple structure, a living fossil
that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian
sedimentary rock
).
The external form of the polyp varies greatly in different cases. The column may be long and slender, or may be so short in the vertical direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be long and filamentous, or short and reduced to mere knobs or warts. They may be simple and unbranched, or they may be feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped. As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic of one of the two classes, Hydrozoa
and Anthozoa
.
In the class Hydrozoa
, the polyps are indeed often very simple, like the common little freshwater
species
of the genus
Hydra
. Anthozoa
n polyps, including the coral
s and sea anemone
s, are much more complex due to the development of a tubular stomodaeum leading inward from the mouth and a series of radial partitions called mesenteries
. Many of the mesenteries project into the enteric cavity but some extend from the body wall to the central stomodaeum.
by the method of budding
. This mode of reproduction may be combined with sexual reproduction
, or may be the sole method by which the polyp produces offspring
, in which case the polyp is entirely without sexual organs. In many cases the buds formed do not separate from the parent but remain in continuity with it, thus forming colonies or stocks, which may reach a great size and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies. The reef
-building corals are polyp-colonies, strengthened by the formation of a firm skeleton.
(Fr. poulpe, ultimately from Greek adverb πολύ (poly, "much") + noun πούς (pous, "foot")), with its circle of writhing arms round the mouth. This comparison, though far-fetched, is certainly more reasonable than the common name "coral-insects" applied to the polyps which form coral.
Zoology
Zoology |zoölogy]]), is the branch of biology that relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct...
is one of two forms found in the phylum
Phylum
In biology, a phylum The term was coined by Georges Cuvier from Greek φῦλον phylon, "race, stock," related to φυλή phyle, "tribe, clan." is a taxonomic rank below kingdom and above class. "Phylum" is equivalent to the botanical term division....
Cnidaria
Cnidaria
Cnidaria is a phylum containing over 9,000 species of animals found exclusively in aquatic and mostly marine environments. Their distinguishing feature is cnidocytes, specialized cells that they use mainly for capturing prey. Their bodies consist of mesoglea, a non-living jelly-like substance,...
, the other being the medusa
Medusa (biology)
In biology, a medusa is a form of cnidarian in which the body is shaped like an umbrella, in contrast with polyps. Medusae vary from bell-shaped to the shape of a thin disk, scarcely convex above and only slightly concave below...
. Polyps are approximately cylindrical in shape and elongated at the axis of the body. In solitary polyps, the aboral
Aboral
In biology, aboral surfaces are surfaces away from or opposite the mouth. The term is a compound of the Latin preposition ā, a, abs, meaning from or away from and the noun ōs, ōris n., meaning mouth. It is also the opposite of oral which is the end containing the mouth of a bilaterally...
end is attached to the substrate
Substrate (biology)
In biology a substrate is the surface a plant or animal lives upon and grows on. A substrate can include biotic or abiotic materials and animals. For example, encrusting algae that lives on a rock can be substrate for another animal that lives on top of the algae. See also substrate .-External...
by means of a disc-like holdfast
Holdfast
A holdfast is a root-like structure that anchors aquatic sessile organisms, such as seaweed, other sessile algae, stalked crinoids, benthic cnidarians, and sponges, to the substrate. ...
called the pedal disc
Pedal disc
The pedal disc is, in anatomy of the sea anemone, the surface opposite to the mouth, and generally serves to attach the anemone to the substrate, or hard surface, upon which it lives. It is composed of a thin tissue plate and is used by the animal to adhere to and move across the surface....
, while in colonies
Colony (biology)
In biology, a colony reference to several individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects live only in colonies...
of polyps it is connected to other polyps, either directly or indirectly. The oral end contains the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....
, and is surrounded by a circlet of tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s.
Classes
In the classClass (biology)
In biological classification, class is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, order, family, genus, and species, with class fitting between phylum and order...
Anthozoa
Anthozoa
Anthozoa is a class within the phylum Cnidaria that contains the sea anemones and corals. Unlike other cnidarians, anthozoans do not have a medusa stage in their development. Instead, they release sperm and eggs that form a planula, which attaches to some substrate on which the cnidarian grows...
, comprising the sea anemones and coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s, the individual is always a polyp; in the class Hydrozoa
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
, however, the individual may be either a polyp or a medusa, with most species undergoing a life cycle
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...
with both a polyp stage and a medusa stage. In class Scyphozoa
Scyphozoa
Scyphozoa is a class within the phylum Cnidaria, sometimes referred to as the "true jellyfish".The class name Scyphozoa comes from the Greek word skyphos , denoting a kind of drinking cup and alluding to the cup shape of the organism....
, the medusa stage is dominant, and the polyp stage may or may not be present, depending on the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
. In those scyphozoans that have the larval planula
Planula
A planula is the free-swimming, flattened, ciliated, bilaterally symmetric larval form of various cnidarian species. The planula forms from the fertilized egg of a medusa, as the case in scyphozoans and some hydrozoans, or from a polyp, as in the case of anthozoans...
metamorphose
Métamorphose
"Métamorphose" - a song by French singer Amanda Lear released in 1989 by Carrere Records.- Song information :"Métamorphose" was the first single from Amanda's French-Italian album Tant qu'il y aura des hommes. The album was actually a re-release of Uomini più uomini, and consisted mostly of down-...
into a polyp, the polyp, also called a "scyphistoma," grows until it develops a stack of plate-like medusae that pinch off and swim away in a process known as strobilation
Strobilation
Strobilation or transverse fission is a form of asexual reproduction consisting of the spontaneous transverse segmentation of the body. It is observed in certain cnidarians and helminths...
. Once strobilation is complete, the polyp may die, or regenerate
Regeneration (biology)
In biology, regeneration is the process of renewal, restoration, and growth that makes genomes, cells, organs, organisms, and ecosystems resilient to natural fluctuations or events that cause disturbance or damage. Every species is capable of regeneration, from bacteria to humans. At its most...
itself to repeat the process again later. With Cubozoans, the planula settles onto a suitable surface, and develops into a polyp. The cubozoan polyp then eventually metamorphoses directly into a Medusa.
Anatomy
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
s. The outer layer is known technically as the ectoderm
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
, the inner layer as the endoderm
Germ layer
A germ layer, occasionally referred to as a germinal epithelium, is a group of cells, formed during animal embryogenesis. Germ layers are particularly pronounced in the vertebrates; however, all animals more complex than sponges produce two or three primary tissue layers...
(or gastroderm). Between ectoderm and endoderm is a supporting layer of structureless gelatinous substance termed mesogloea, secreted by the cell layers of the body wall. The mesogloea may be a very thin layer, or may reach a fair thickness, and then sometimes contains skeletal
Endoskeleton
An endoskeleton is an internal support structure of an animal, composed of mineralized tissue. Endoskeleton develops within the skin or in the deeper body tissues. The vertebrate is basically an endoskeleton made up of two types of tissues . During early embryonic development the endoskeleton is...
elements formed by cells which have migrated
Cell migration
Cell migration is a central process in the development and maintenance of multicellular organisms. Tissue formation during embryonic development, wound healing and immune responses all require the orchestrated movement of cells in particular directions to specific locations...
into it from the ectoderm.
The sac-like body built up in this way is attached usually to some firm object by its blind end, and bears at the upper end the mouth which is surrounded by a circle of tentacle
Tentacle
A tentacle or bothrium is one of usually two or more elongated flexible organs present in animals, especially invertebrates. The term may also refer to the hairs of the leaves of some insectivorous plants. Usually, tentacles are used for feeding, feeling and grasping. Anatomically, they work like...
s which resemble glove fingers. The tentacles are organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s which serve both for the tactile sense and for the capture of food. Polyps extend their tentacles, particularly at night, containing coiled like stinging nettle-cells or nematocysts which pierce and poison and firmly hold living prey paralysing or killing them which include copepods and fish larvae. Longitudinal muscular fibrils formed from the cells of the ectoderm allow tentacles to contract when conveying the food to the mouth. Similarly, circularly disposed muscular fibrils formed from the endoderm permit tentacles to be protract or thrust out once they are contracted. These muscle fibres belonging to the same two systems, thus allows the whole body to retract or protrude outwards.
We can distinguish therefore in the body of a polyp the column, circular or oval in section, forming the trunk, resting on a base or foot and surmounted by the crown of tentacles, which enclose an area termed the peristome
Peristome
The word peristome is derived from the Greek peri, meaning 'around' or 'about', and stoma, 'mouth'. It is a term used to describe various anatomical features that surround an opening to an organ or structure. The term is used in plants and invertebrate animals, such as in describing the shells of...
, in the centre of which again is the mouth. As a rule there is no other opening to the body except the mouth, but in some cases excretory pore
Pore
- Animal biology and microbiology :* Sweat pore, an anatomical structure of the skin of humans used for secretion of sweat* Canal pore, an anatomical structure that is part of the lateral line sense system of some aquatic organisms...
s are known to occur in the foot, and pores may occur at the tips of the tentacles. Thus it is seen that a polyp is an animal of very simple structure, a living fossil
Living fossil
Living fossil is an informal term for any living species which appears similar to a species otherwise only known from fossils and which has no close living relatives, or a group of organisms which have long fossil records...
that has not changed significantly for about half a billion years (per generally accepted dating of Cambrian
Cambrian
The Cambrian is the first geological period of the Paleozoic Era, lasting from Mya ; it is succeeded by the Ordovician. Its subdivisions, and indeed its base, are somewhat in flux. The period was established by Adam Sedgwick, who named it after Cambria, the Latin name for Wales, where Britain's...
sedimentary rock
Sedimentary rock
Sedimentary rock are types of rock that are formed by the deposition of material at the Earth's surface and within bodies of water. Sedimentation is the collective name for processes that cause mineral and/or organic particles to settle and accumulate or minerals to precipitate from a solution....
).
The external form of the polyp varies greatly in different cases. The column may be long and slender, or may be so short in the vertical direction that the body becomes disk-like. The tentacles may number many hundreds or may be very few, in rare cases only one or two. They may be long and filamentous, or short and reduced to mere knobs or warts. They may be simple and unbranched, or they may be feathery in pattern. The mouth may be level with the surface of the peristome, or may be projecting and trumpet-shaped. As regards internal structure, polyps exhibit two well-marked types of organization, each characteristic of one of the two classes, Hydrozoa
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
and Anthozoa
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
.
In the class Hydrozoa
Hydrozoa
Hydrozoa are a taxonomic class of very small, predatory animals which can be solitary or colonial and which mostly live in saltwater. A few genera within this class live in freshwater...
, the polyps are indeed often very simple, like the common little freshwater
Freshwater
Fresh water is naturally occurring water on the Earth's surface in ice sheets, ice caps, glaciers, bogs, ponds, lakes, rivers and streams, and underground as groundwater in aquifers and underground streams. Fresh water is generally characterized by having low concentrations of dissolved salts and...
species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
of the genus
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Hydra
Hydra (genus)
Hydra is a genus of simple fresh-water animal possessing radial symmetry. Hydras are predatory animals belonging to the phylum Cnidaria and the class Hydrozoa. They can be found in most unpolluted fresh-water ponds, lakes, and streams in the temperate and tropical regions and can be found by...
. Anthozoa
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
n polyps, including the coral
Coral
Corals are marine animals in class Anthozoa of phylum Cnidaria typically living in compact colonies of many identical individual "polyps". The group includes the important reef builders that inhabit tropical oceans and secrete calcium carbonate to form a hard skeleton.A coral "head" is a colony of...
s and sea anemone
Sea anemone
Sea anemones are a group of water-dwelling, predatory animals of the order Actiniaria; they are named after the anemone, a terrestrial flower. Sea anemones are classified in the phylum Cnidaria, class Anthozoa, subclass Zoantharia. Anthozoa often have large polyps that allow for digestion of larger...
s, are much more complex due to the development of a tubular stomodaeum leading inward from the mouth and a series of radial partitions called mesenteries
Mesentery
In anatomy, the mesentery is the double layer of peritoneum that suspends the jejunum and ileum from the posterior wall of the abdomen. Its meaning, however, is frequently extended to include double layers of peritoneum connecting various components of the abdominal cavity.-Mesentery :The...
. Many of the mesenteries project into the enteric cavity but some extend from the body wall to the central stomodaeum.
Reproduction
It is an almost universal attribute of polyps to reproduce asexuallyAsexual reproduction
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis, ploidy reduction, or fertilization. A more stringent definition is agamogenesis which is reproduction without...
by the method of budding
Budding
Budding is a form of asexual reproduction in which a new organism grows on another one. The new organism remains attached as it grows, separating from the parent organism only when it is mature. Since the reproduction is asexual, the newly created organism is a clone and is genetically identical...
. This mode of reproduction may be combined with sexual reproduction
Sexual reproduction
Sexual reproduction is the creation of a new organism by combining the genetic material of two organisms. There are two main processes during sexual reproduction; they are: meiosis, involving the halving of the number of chromosomes; and fertilization, involving the fusion of two gametes and the...
, or may be the sole method by which the polyp produces offspring
Offspring
In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...
, in which case the polyp is entirely without sexual organs. In many cases the buds formed do not separate from the parent but remain in continuity with it, thus forming colonies or stocks, which may reach a great size and contain a vast number of individuals. Slight differences in the method of budding produce great variations in the form of the colonies. The reef
Reef
In nautical terminology, a reef is a rock, sandbar, or other feature lying beneath the surface of the water ....
-building corals are polyp-colonies, strengthened by the formation of a firm skeleton.
Etymology
The name polyp was given to these organisms from their supposed resemblance to an octopusOctopus
The octopus is a cephalopod mollusc of the order Octopoda. Octopuses have two eyes and four pairs of arms, and like other cephalopods they are bilaterally symmetric. An octopus has a hard beak, with its mouth at the center point of the arms...
(Fr. poulpe, ultimately from Greek adverb πολύ (poly, "much") + noun πούς (pous, "foot")), with its circle of writhing arms round the mouth. This comparison, though far-fetched, is certainly more reasonable than the common name "coral-insects" applied to the polyps which form coral.