- Internal space for digestion is the gastrovascular cavity
- Gastrovascular cavity has one opening, the mouth
- Exoskeleton of chitin
- Are almoust entirely marine and predators
- Sexual reproduction produces the planula larvae
- Two body forms, a polyp and medusea
- Presence of stinging cells called Cnidocytes
- 7 orders consisting of 2700 species
Hydrozoa consist of several marine organisms existinting in
all three layers of the marine habitat. Some Hydrozoans live on the
the surface(velella and Physalia) floating with large sail like
structures above water for locomotion and long tentacles with
nematocytes or stinging cells below the surface to catch food. A
second group live in the middle water zone or pelagic region. These
organisms such as the Chelia and Bougainvillae umbrella shaped with
long tentacles emerging from the ventral region and move by jet
propelling themselves up and passively sinking down. The third region
for the Hydrozoans is the bottom where they anchor themselves to the
substrate. These Hydrozoans are the orders Milleporina and
Stylasterina which are corals and form an internal, epidermal skeleton
of calcareous. These organisms can grow very large covering mass
amounts of ocean substrate. Another sessile Hydrozoa is the Hydra
which is truely unique among the Hydrozoans. The Hydra is solitary,
lacks a medusoid phase and feeds with long tentacles that extend from
around the mouth.
Some Interesting Facts:
- Hydrozoans have many cell types, but form only two tissue
types for their bodies, epidermus and gastrodermis. Their bodies can
range in size from the planula larvae which is measured in micrometers
to massive colonial corals covering imense amounts of area.
- Though these organisms feed on smaller organisms, their
digestive system is a simple gastrovascular cacity lined with
flagellated cells that circulate food in with the filtering of water.
- The use of Cnidocytes enable the Hydrozoans to paralyze prey
as well as ward off predators. There stinging cells are jettosoned
after use and interstitial cells regenerate new ones. Developing
Cnidocytes are called Cnidoblasts.
Some Pertinent Books Found In Lamson Library:
- Parker, Sybil, Editor in Chief. 1982. Synopsis and Classification
Organisms: vol 1. New York: McGraw-Hill Inc.
- Lenhoff, Howard M. 1983. Hydra: Research Methods. New York:
- Lenhoff, Howard M. and Loomis, Farnsworth W. 1961. The Biology
Hydra and Some Other Coelenterates. Florida: Univ. of Miami Press.
(Philip N. Bassignani)