General characteristics of the type of coelenterates. Class Hydroids. SUMMARY
- Cnidarians are aquatic, radially symmetrical animals with tentacles encircling the mouth at one end of the body. The mouth is the only opening into the gut cavity.
- Cnidarians exhibit two body forms: the me¬dusa, which is adapted for a pelagic existence, and the polyp, which is adapted for an attached, benthic existence. Colonial organization has evolved in many polypoid groups.
- The body wall consists of an outer epidermis, an inner gastrodermis, and an intervening mesoglea. The latter may be thin or thick, acellularor cellular.
- Cnidarians are primitive in their lack of orl gans theii lack oi fully differentiated epithelil and muscle cells, and the diploblastic origin of Л adult body.
- Most feed on zooplankton, although son utilize larger animals and some are suspension! feeders on fine particulate matter. Prey is caugbJ with the tentacles and immobilized by ехркші cells, called cnidocytes, which are unique to tfj phylum. Digestion is initially extracellular, thd intracellular.
- The neurons arc usually arranged as a nervel net at the base of the epidermal and gastrodenol layers, and impulse transmission tends to be ] dialing. Synaptic junctions are common^ nonpolarized.
- A ciliated, free-swimming stereogastrula, I called the planula larva, occurs in the life cycleif most cnidarians.
The class Hydrozoa contains about 2700 species of common cnidarians, but because of their small size and plantlike appearance, the layman is largely unaware of their existence. A considerable part of the marine growth attached to rocks, shells, and wharf pilings, usually dismissed as "seaweed," is frequently composed of hydrozoan cnidarians.
The few known freshwater cnidarians belong to to the class Hydrozoa. They include the hydras and some small, freshwater jellyfish.
Hydrozoans display either the polypoid or the medusoid structure, and some species pass through both forms in their life cycle. Three characteristic unite the members of this class. The mesoglea is never cellular; the gastrodermis lacks cnidocytes and the gonads are epidermal, or if gastrodermal, the eggs and sperm are shed directly to the outside and not into the gastrovascular cavity.