Class Hydrozoa
These organism are small and very common cnidarians. These organisms are mainly marine, but there are some freshwater specimen. Organisms in this class alternate between medusa and polyp body forms. But sometimes the medusa stage is lost and the polyp stage is sometimes small. Examples of organisms in this class are shown above.
Distinguishing features:
Polyps here are mainly colonial, each indicidual polyp has a specialized job, such as feeding (gastrozooids) or reproduction (gonozooid). Obelia (alternate between medusa and polyp) are a common type of cnidaria in this class. In obelia the gastrozooid develops from a planula. Gastrozooids have tentacles, feed on small organisms, and secrete skeleton protein and chitin (perisac). A colony of polyps is able to grow from an original gastrozooid, new polyps emerge through budding and branch of from the original polyp. These colonies have a continuous gastrovascular cavity. Gonozooids, or gonangium, is a polyp that can be produced from this colony. Gonozooids are reproductive (asexual) polyps, reproduce through budding.
Gonionemus (dominantly medusae) is a type of cnidarian, also in class Hydrozoa. This organism has a velum, projection in the margin of the medusa. The velum concentrates water being expelled from under the medusa's oral surface, acting as a jet-propulsion system. The mouth is at the end of the manubrium, tube-like structure that hangs from the medusa's oral surface. From the manubrium extends the gastovascular cavity into four radial canals, these canals extend to the margins of the medusa. A encircling canal connects the canals at the margins of the medusa. The gonionemus has a nerve ring, in addition to a nerve net, which is a concentration of nerve cells. The nerve ring is able to coordinate movements when swimming. Statocyst, small sac surrounded by a calcium concretion called a statolith are embedded in the margins of a medusa. When the gonionemus tilts, the statolith moves in response to the pull of gravity, which starts nerve impulses that can change the way this organisms is moving.
Distinguishing features:
- Nematocysts are only in the epidermis.
- Gametes are epidermal and and released to the outside, not to the gastrovascular cavity.
- Mesoglea mainly acellular.
Polyps here are mainly colonial, each indicidual polyp has a specialized job, such as feeding (gastrozooids) or reproduction (gonozooid). Obelia (alternate between medusa and polyp) are a common type of cnidaria in this class. In obelia the gastrozooid develops from a planula. Gastrozooids have tentacles, feed on small organisms, and secrete skeleton protein and chitin (perisac). A colony of polyps is able to grow from an original gastrozooid, new polyps emerge through budding and branch of from the original polyp. These colonies have a continuous gastrovascular cavity. Gonozooids, or gonangium, is a polyp that can be produced from this colony. Gonozooids are reproductive (asexual) polyps, reproduce through budding.
Gonionemus (dominantly medusae) is a type of cnidarian, also in class Hydrozoa. This organism has a velum, projection in the margin of the medusa. The velum concentrates water being expelled from under the medusa's oral surface, acting as a jet-propulsion system. The mouth is at the end of the manubrium, tube-like structure that hangs from the medusa's oral surface. From the manubrium extends the gastovascular cavity into four radial canals, these canals extend to the margins of the medusa. A encircling canal connects the canals at the margins of the medusa. The gonionemus has a nerve ring, in addition to a nerve net, which is a concentration of nerve cells. The nerve ring is able to coordinate movements when swimming. Statocyst, small sac surrounded by a calcium concretion called a statolith are embedded in the margins of a medusa. When the gonionemus tilts, the statolith moves in response to the pull of gravity, which starts nerve impulses that can change the way this organisms is moving.
Gonionemus (dominantly medusae) is a type of cnidarian, also in class Hydrozoa. This organism has a velum, projection in the margin of the medusa. The velum concentrates water being expelled from under the medusa's oral surface, acting as a jet-propulsion system. The mouth is at the end of the manubrium, tube-like structure that hangs from the medusa's oral surface. From the manubrium extends the gastovascular cavity into four radial canals, these canals extend to the margins of the medusa. A encircling canal connects the canals at the margins of the medusa. The gonionemus has a nerve ring, in addition to a nerve net, which is a concentration of nerve cells. The nerve ring is able to coordinate movements when swimming. Statocyst, small sac surrounded by a calcium concretion called a statolith are embedded in the margins of a medusa. When the gonionemus tilts, the statolith moves in response to the pull of gravity, which starts nerve impulses that can change the way this organisms is moving.