Hydrozoa
Overview
Hydrozoa are a taxonomic class
Class (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...

 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. Hydrozoans are related to jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 and corals and belong to the phylum 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,...

.

Some examples of hydrozoans are the Freshwater Jelly (Craspedacusta sowerbyi), the freshwater polyps (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...

), Obelia
Obelia
Obelia is a genus in the class Hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle...

, the Portuguese Man o' War
Portuguese Man o' War
The Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...

 (Physalia physalis), the chondrophore
Chondrophore
The chondrophores or porpitids are a small and very unusual group of hydrozoans today classified as family Porpitidae. Though it derives from an outdated name for this lineage , some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the three genera contained...

s (Porpitidae), "air fern
Air fern
Neptune plant or Air fern is a name given to a product that is composed of a species of marine animal called Sertularia argentea, also known as the "sea fir"....

" (Sertularia argenta) and the pink-hearted hydroid
Tubularia
Tubularia is a genus of hydroids that appear to be furry pink tufts or balls at the end of long strings, thus causing them to be sometimes be called "pink-mouthed" or "pink-hearted" hydroids. T. larynx is described as:The stems are tubular, with a yellowish coloured tegument and are branched at the...

s (Tubularia).
Most hydrozoan species include both a hydroid
Polyp
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...

 and a medusoid
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...

 stage in their life cycle, although there are a number that have only one or the other.
Encyclopedia
Hydrozoa are a taxonomic class
Class (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...

 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. Hydrozoans are related to jellyfish
Jellyfish
Jellyfish are free-swimming members of the phylum Cnidaria. Medusa is another word for jellyfish, and refers to any free-swimming jellyfish stages in the phylum Cnidaria...

 and corals and belong to the phylum 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,...

.

Some examples of hydrozoans are the Freshwater Jelly (Craspedacusta sowerbyi), the freshwater polyps (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...

), Obelia
Obelia
Obelia is a genus in the class Hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle...

, the Portuguese Man o' War
Portuguese Man o' War
The Portuguese Man o' War , also known as the Portuguese man-of-war, man-of-war, or bluebottle, is a jelly-like marine invertebrate of the family Physaliidae...

 (Physalia physalis), the chondrophore
Chondrophore
The chondrophores or porpitids are a small and very unusual group of hydrozoans today classified as family Porpitidae. Though it derives from an outdated name for this lineage , some still find the term "chondrophore" useful as a synonym to "porpitid" in discussions of the three genera contained...

s (Porpitidae), "air fern
Air fern
Neptune plant or Air fern is a name given to a product that is composed of a species of marine animal called Sertularia argentea, also known as the "sea fir"....

" (Sertularia argenta) and the pink-hearted hydroid
Tubularia
Tubularia is a genus of hydroids that appear to be furry pink tufts or balls at the end of long strings, thus causing them to be sometimes be called "pink-mouthed" or "pink-hearted" hydroids. T. larynx is described as:The stems are tubular, with a yellowish coloured tegument and are branched at the...

s (Tubularia).

Anatomy

Most hydrozoan species include both a hydroid
Polyp
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...

 and a medusoid
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...

 stage in their life cycle, although there are a number that have only one or the other. For example, 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...

has no medusoid stage, while Liriope lacks the hydroid stage.

Polyps

The hydroid form is usually colonial, with multiple polyps connected together by tubelike hydrocauli. The hollow cavity in the middle of the polyp extends into the associated hydrocaulus, so that all the individuals of the colony are intimately connected. Where the hydrocaulus runs along the substrate it form a horizontal root-like stolon
Stolon
In biology, stolons are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external skeletons.-In botany:...

 that anchors the colony to the ground.

The colonies are generally small, no more than a few centimeters across, and may have a tree-like or fan-like appearance, depending on species. The polyps themselves are usually tiny, although some non-colonial species are much larger, reaching 6 to 9 cm (2.4 to 3.5 in), or, in the case of the deep sea Branchiocerianthus, a remarkable 2 metres (6.6 ft).

The hydrocaulus is usually surrounded by a sheath of chitin
Chitin
Chitin n is a long-chain polymer of a N-acetylglucosamine, a derivative of glucose, and is found in many places throughout the natural world...

 and proteins called the perisarc. In some species, this extends upwards to also enclose part of the polyps, in some cases including a closable lid through which the polyp may extend its tentacles.

In any given colony, the majority of polyps are specialized for feeding. These have a more or less cylindrical body with a terminal mouth on a raised protuberance called the hypostome
Hypostome
A hypostome , is a calcified harpoon-like structure near the mouth area of certain parasitic arthropods including ticks and mites , that allows them to anchor themselves firmly in place on a host mammal while sucking blood...

, surrounded by a number of tentacles. The polyp contains a central cavity, in which initial digestion takes place. Partially digested food may then be passed into the hydrocaulus for distribution around the colony and completion of the digestion process. Unlike some other cnidarian groups, the lining of the central cavity lacks stinging nematocysts, which are found only on the tentacles and outer surface.

All colonial hydrozoans also include some polyps specialized for reproduction. These lack tentacles and contain numerous buds from which the medusoid stage of the life-cycle is produced. The arrangement and type of these reproductive polyps varies considerably between different groups.

In addition to these two basic types of polyp, a few colonial species have other specialized forms. In some, defensive polyps are found, armed with large numbers of stinging cells. In others, one polyp may develop as a large float, from which the other polyps hang down, allowing the colony to drift in open water instead of being anchored to a solid surface.

Medusae

The medusae of hydrozoans are smaller than those of typical jellyfish, ranging from 0.5 to 6 cm (0.196850393700787 to 2.4 ) in diameter. Although most hydrozoans have a medusoid stage, this is not always free-living, and in many species, exists solely as a sexually reproducing bud on the surface of the hydroid colony. Sometimes these medusoid buds may be so degenerate as to entirely lack tentacles or mouths, essentially consisting of an isolated gonad
Gonad
The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

.

The body consists of a dome-like umbrella ringed by tentacles. A tube-like structure hangs down from the centre of the umbrella, and includes the mouth at its tip. Most hydrozoan medusae have just four tentacles, although a number of exceptions exist. Stinging cells are found on the tentacles and around the mouth.

The mouth leads into a central stomach cavity. Four radial canals connect the stomach to an, additional, circular canal running around the base of the bell, just above the tentacles. Striated muscle fibres also line the rim of the bell, allowing the animal to move along by alternately contracting and relaxing its body. An additional shelf of tissue lies just inside the rim, narrowing the aperture at the base of the umbrella, and thereby increasing the force of the expelled jet of water.

The nervous system is unusually advanced for cnidarians, or even than in the polyps of the same species. Two nerve rings lie close to the margin of the bell, and send fibres into the muscles and tentacles. The genus Sarsia has even been reported to possess organised ganglia
Ganglion
In anatomy, a ganglion is a biological tissue mass, most commonly a mass of nerve cell bodies. Cells found in a ganglion are called ganglion cells, though this term is also sometimes used to refer specifically to retinal ganglion cells....

. Numerous sense organs are closely associated with the nerve rings. Mostly these are simple sensory nerve endings, but they also include statocyst
Statocyst
The statocyst is a balance sensory receptor present in some aquatic invertebrates, including bivalves, cnidarians, echinoderms, cephalopods, and crustaceans. A similar structure is also found in Xenoturbella. The statocyst consists of a sac-like structure containing a mineralised mass and numerous...

s and primitive light-sensitive ocelli.

Life cycle

Hydroid
Hydroid
-Marine biology:Hydroids are a life stage for most animals of class Hydrozoa, small predators related to jellyfish.-Botany:In mosses, hydroids form the innermost layer of the stem of long, colourless, thin walled cells of small diameter.The cells are dead and lack protoplasm.They function as water...

 colonies are usually dioecious
Dioecious
Dioecy is the property of a group of biological organisms that have males and females, but not members that have organs of both sexes at the same time. I.e., those whose individual members can usually produce only one type of gamete; each individual organism is thus distinctly female or male...

, which means that they have separate sexes - all the polyps in each colony are either male or female, but not usually both sexes in the same colony. In some species, the reproductive polyps, known as gonozooids (or "gonotheca" in thecate hydrozoans
Leptomedusae
Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"...

) bud off asexually-produced medusae. These tiny, new medusae (which are either male or female) mature and spawn, releasing gamete
Gamete
A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s freely into the sea in most cases. Zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...

s become free-swimming planula larvae
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...

 or actinula larvae that either settle on a suitable 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...

 (in the case of planulae), or swim and develop into another medusae or polyp directly (actinulae). Colonial hydrozoans include siphonophore colonies, Hydractinia, Obelia
Obelia
Obelia is a genus in the class Hydrozoa, which consists of mainly marine and some freshwater animal species and have both the polyp and medusa stages in their life cycle...

, and many others.

The medusa stage, if present, is the sexually-reproductive life cycle phase (that is, in hydrozoan species that have both polyp and medusa generations). Medusae of these species of Hydrozoa are known as "hydromedusae". Most hydromedusae have shorter life spans than the larger 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....

n jellyfish. Some species of hydromedusae release gametes shortly after they are themselves released from the hydroids (as in the case of fire coral
Fire coral
Fire corals are colonial marine organisms that look rather like real coral. However they are technically not corals; they are actually more closely related to jellyfish and other stinging anemones...

s), living only a few hours, while other species of hydromedusae grow and feed in the plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

 for months, spawning daily for many days before their supply of food or other water conditions deteriorate and cause their demise.

Systematics

Hydrozoan systematics
Systematics
Biological systematics is the study of the diversification of terrestrial life, both past and present, and the relationships among living things through time. Relationships are visualized as evolutionary trees...

 is highly complex. Several approaches for expressing their interrelationships were proposed and heavily contested since the late 19th century, but in more recent times a consensus seems to be emerging.

For long, the hydrozoans were divided into a number of order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

s, according to their mode of growth and reproduction. Most famous among these was probably the assemblage called "Hydroida
Hydroida
Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives...

", but this group is apparently paraphyletic, united by plesiomorphic (ancestral) traits. Other such orders were the Anthoathecatae, Actinulidae, Laingiomedusae, Polypodiozoa, Siphonophora
Siphonophora
Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a...

 and Trachylina.

As far as can be told from the molecular and morphological
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....

 data at hand, the Siphonophora for example were just highly specialized "hydroids," whereas the Limnomedusae - presumed to be a "hydroid" suborder - were simply very primitive hydrozoans and not closely related to the other "hydroids." Therefore, today the hydrozoans are at least tentatively divided into two subclass
Class (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...

es, the Leptolinae
Leptolinae
Leptolinae are a cnidarian subclass of the Hydrozoa. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank...

 (containing the bulk of the former "Hydroida" and the Siphonophora) and the Trachylinae
Trachylinae
Trachylinae are a cnidarian subclass of the Hydrozoa. They are placed at order rank in many older classifications, and limited to contain the Narcomedusae and Trachymedusae...

, containing the others (including the Limnomedusae). The monophyly
Monophyly
In common cladistic usage, a monophyletic group is a taxon which forms a clade, meaning that it contains all the descendants of the possibly hypothetical closest common ancestor of the members of the group. The term is synonymous with the uncommon term holophyly...

 of several of the presumed orders in each subclass is still in need of verification.

In any case, according to this classification, the hydrozoans can be subdivided as follows, with taxon
Taxon
|thumb|270px|[[African elephants]] form a widely-accepted taxon, the [[genus]] LoxodontaA taxon is a group of organisms, which a taxonomist adjudges to be a unit. Usually a taxon is given a name and a rank, although neither is a requirement...

 names emended to end in "-ae":

Class Hydrozoa
  • Subclass Leptolinae
    Leptolinae
    Leptolinae are a cnidarian subclass of the Hydrozoa. They contain the bulk of the paraphyletic "Hydroida" which were one of the main groupings of the Hydrozoa in older classifications and were placed at order rank...

    • Order Anthomedusae
      Anthomedusae
      Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria...

       (= Anthoathecata(e), Athecata(e), Stylasterina(e)) - includes Laingoimedusae but monophyly requires verification
    • Order Leptomedusae
      Leptomedusae
      Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"...

       (= Leptothecata(e), Thecaphora(e), Thecata(e))
    • Order Siphonophorae
  • Subclass Trachylinae
    Trachylinae
    Trachylinae are a cnidarian subclass of the Hydrozoa. They are placed at order rank in many older classifications, and limited to contain the Narcomedusae and Trachymedusae...

    • Order Actinulidae
    • Order Limnomedusae - monophyly requires verification; tentatively placed here
    • Order Narcomedusae
      Narcomedusae
      Narcomedusae is an order of hydrozoans in the subclass Trachylinae.-Characteristics:Members of this order do not normally have a polyp stage. The medusa has a dome-shaped bell with thin sides. The tentacles are attached above the lobed margin of the bell with usually a gastric pouch above each....

    • Order Trachymedusae
      Trachymedusae
      Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 families, of which the Rhopalonematidae are by far the most diverse. They only reproduce sexually as medusa, never forming polyps...

       - monophyly requires verification


ITIS
Itis
Itis may refer to* Integrated Taxonomic Information System, a partnership designed to provide consistent and reliable information on the taxonomy of biological species...

 uses the same system but unlike here does not use the oldest available names for many groups.

In addition, there exists a weird cnidarian parasite, Polypodium hydriforme, which lives inside its host's cell
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. It is sometimes placed in the Hydrozoa, but actually its relationships are better treated as unresolved for the time being - a somewhat controversial 18S rRNA sequence
DNA sequence
The sequence or primary structure of a nucleic acid is the composition of atoms that make up the nucleic acid and the chemical bonds that bond those atoms. Because nucleic acids, such as DNA and RNA, are unbranched polymers, this specification is equivalent to specifying the sequence of...

 analysis found it to be closer to Myxozoa
Myxozoa
The Myxozoa are a group of parasitic animals of aquatic environments. Over 1300 species have been described and many have a two-host lifecycle, involving a fish and an annelid worm or bryozoan. The average size of a Myxosporea spore usually ranges from 10 μm to 20 μm and Malacosporea up...

. It was traditionally placed in its own class
Class (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...

 Polypodiozoa and this view is presently often seen to reflect the uncertainties surrounding this highly distinct animal.

Other classifications

Some of the more widespread classification systems for the Hydrozoa are listed below. Though they are often found in seemingly authoritative Internet sources and databases, they do not agree with the currently available data. Especially the presumed phylogenetic distinctness of the Siphonophora
Siphonophora
Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a...

 is a major flaw that was corrected only recently.

The obsolete classification mentioned above was as follows:
  • Order Actinulidae
  • Order Anthoathecatae
  • Order Hydroida
    Hydroida
    Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives...

    • Suborder Anthomedusae
      Anthomedusae
      Anthomedusae or Anthomedusa, the athecate hydroids, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria...

    • Suborder Leptomedusae
      Leptomedusae
      Leptomedusae or Leptomedusa, commonly called thecate hydroids, are a cnidarian order in the subclass Leptolinae. They were formerly placed at suborder rank in the paraphyletic "Hydroida"...

    • Suborder Limnomedusae
  • Order Laingiomedusae
  • Order Polypodiozoa
  • Order Siphonophora
    Siphonophora
    Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a...


  • Order Trachylina
    • Suborder Narcomedusae
      Narcomedusae
      Narcomedusae is an order of hydrozoans in the subclass Trachylinae.-Characteristics:Members of this order do not normally have a polyp stage. The medusa has a dome-shaped bell with thin sides. The tentacles are attached above the lobed margin of the bell with usually a gastric pouch above each....

    • Suborder Trachymedusae
      Trachymedusae
      Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 families, of which the Rhopalonematidae are by far the most diverse. They only reproduce sexually as medusa, never forming polyps...



A very old classification that is sometimes still seen is:
  • Order Hydroida
    Hydroida
    Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives...

  • Order Milleporina
  • Order Siphonophorida
  • Order Stylasterina (= Anthomedusae)
  • Order Trachylinida


Catalogue of Life
Catalogue of Life
The Catalogue of Life, started in June 2001 by Species 2000 and Integrated Taxonomic Information System , is planned to become a comprehensive catalogue of all known species of organisms on Earth by the year 2011. 66 taxonomic databases with contributions from more than 3,000 specialists from...

 uses the following:
  • Order Actinulida
  • Order Anthoathecata (= Anthomedusae)
  • Order Hydroida
    Hydroida
    Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives...


  • Order Laingiomedusae
  • Order Leptothecata (= Leptomedusae)
  • Order Limnomedusae
  • Order Narcomedusae
    Narcomedusae
    Narcomedusae is an order of hydrozoans in the subclass Trachylinae.-Characteristics:Members of this order do not normally have a polyp stage. The medusa has a dome-shaped bell with thin sides. The tentacles are attached above the lobed margin of the bell with usually a gastric pouch above each....

  • Order Siphonophora
    Siphonophora
    Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a...

  • Order Trachymedusae
    Trachymedusae
    Trachymedusae are an order in the cnidarian class Hydrozoa. They contain some 50 species, divided among about 30 genera and 5 families, of which the Rhopalonematidae are by far the most diverse. They only reproduce sexually as medusa, never forming polyps...



Animal Diversity Web
Animal Diversity Web
Animal Diversity Web is an online database that collects the natural history, classification, species characteristics, conservation biology, and distribution information of thousands of species of animals...

 uses the following:
  • Order Actinulida
  • Order Capitata
  • Order Chondrophora
  • Order Filifera
  • Order Hydroida
    Hydroida
    Hydroida is an obsolete cnidarian order which united such animals as hydras, hydromedusae, and many marine attached hydroids. However, the group is paraphyletic and not composed from close relatives...

  • Order Siphonophora
    Siphonophora
    Siphonophorae or Siphonophora, the siphonophores, are an order of the Hydrozoa, a class of marine invertebrates belonging to the phylum Cnidaria. They are colonial, but the colonies can superficially resemble jellyfish; although they appear to be a single organism, each specimen is actually a...


Hydra, a freshwater genus

The most widely-known and researched freshwater hydrozoan is 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...

, which is found in slow-moving waters.

Hydra has a 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....

 composed of gland cells that helps it attach to substrates, and like all cnidarians uses nematocysts, or "stinging cells," to disable its prey. Hydra eat small crustacean
Crustacean
Crustaceans form a very large group of arthropods, usually treated as a subphylum, which includes such familiar animals as crabs, lobsters, crayfish, shrimp, krill and barnacles. The 50,000 described species range in size from Stygotantulus stocki at , to the Japanese spider crab with a leg span...

s (such as brine shrimp), insect
Insect
Insects are a class of living creatures within the arthropods that have a chitinous exoskeleton, a three-part body , three pairs of jointed legs, compound eyes, and two antennae...

 larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...

e, and annelid
Annelid
The annelids , formally called Annelida , are a large phylum of segmented worms, with over 17,000 modern species including ragworms, earthworms and leeches...

 worms. Hydra may reproduce sexually, through the spawning of sperm (and thus insemination of eggs on the female body column), or through asexual reproduction
Asexual 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...

(budding).

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