Class Hydrozoa

Main attributes:
  1. Internal space for digestion is the gastrovascular cavity
  2. Gastrovascular cavity has one opening, the mouth
  3. Exoskeleton of chitin
  4. Are almoust entirely marine and predators
  5. Sexual reproduction produces the planula larvae
  6. Two body forms, a polyp and medusea
  7. Presence of stinging cells called Cnidocytes
  8. 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:

Some Pertinent Books Found In Lamson Library:
  1. Parker, Sybil, Editor in Chief. 1982. Synopsis and Classification of Living Organisms: vol 1. New York: McGraw-Hill Inc.
  2. Lenhoff, Howard M. 1983. Hydra: Research Methods. New York: Plenum Press.
  3. Lenhoff, Howard M. and Loomis, Farnsworth W. 1961. The Biology of Hydra and Some Other Coelenterates. Florida: Univ. of Miami Press.
    (Philip N. Bassignani)