Hydroids are a
life stage for most animals of the class
Hydrozoa
Hydrozoa (hydrozoans; ) are a taxonomic class of individually very small, predatory animals, some solitary and some colonial, most of which inhabit saline water. The colonies of the colonial species can be large, and in some cases the special ...
, small predators related to
jellyfish
Jellyfish and sea jellies are the informal common names given to the medusa-phase of certain gelatinous members of the subphylum Medusozoa, a major part of the phylum Cnidaria. Jellyfish are mainly free-swimming marine animals with umbrel ...
.
Some hydroids such as the freshwater ''
Hydra'' are solitary, with the
polyp attached directly to the
substrate. When these produce buds, they become detached and grow on as new individuals.
The majority of hydroids are
colonial. The original polyp is anchored to a solid
substrate and forms a bud which remains attached to its parent. This in turn buds and in this way a stem is formed. The arrangement of polyps and the branching of the stem is characteristic of the species.
Some species have the polyps budding directly off the
stolon
In biology, stolons (from Latin '' stolō'', genitive ''stolōnis'' – "branch"), also known as runners, are horizontal connections between organisms. They may be part of the organism, or of its skeleton; typically, animal stolons are external ...
which roots the colony. The polyps are connected by
epidermis
The epidermis is the outermost of the three layers that comprise the skin, the inner layers being the dermis and hypodermis. The epidermis layer provides a barrier to infection from environmental pathogens and regulates the amount of water relea ...
which surrounds a gastrovascular cavity. The epidermis secretes a
chitin
Chitin ( C8 H13 O5 N)n ( ) is a long-chain polymer of ''N''-acetylglucosamine, an amide derivative of glucose. Chitin is probably the second most abundant polysaccharide in nature (behind only cellulose); an estimated 1 billion tons of chit ...
ous skeleton which supports the stem and in some hydroids, the skeleton extends into a cup shape surrounding the polyp.
Most of the polyps are
gastrozooids or feeding polyps, but some are specialised reproductive structures known as
gonozooid In biology, Gonozooids are any of the reproductive individuals of Tunicates, Bryozoan, or Hydrozoan colonies that produce gametes. Gonozooids may play a role in labour division or in alternation of generations. A gonozooid typically has hardly an ...
s. In some species, further specialised
zooid
A zooid or zoöid is a single animal that is part of a colonial animal. This lifestyle has been adopted by animals from separate unrelated taxa. Zooids are multicellular; their structure is similar to that of other solitary animals. The zooi ...
s are formed.
References
{{reflist, 33em
Zoology
Hydrozoa