Definition

1. Noun (Biology): A class of coelenterates (cnidarians) characterized by a life cycle that typically involves alternation of generations. This cycle usually includes a sessile, often colonial, polyp stage (the hydroid phase) that reproduces asexually by budding to produce a free-swimming medusa stage (the medusoid phase). The class includes organisms such as hydras, Portuguese man o' war, and many types of jellyfishes.

Usage

• Noun:
  • The class Hydrozoa is distinguished by its complex life cycle.
  • Many organisms in the class Hydrozoa are marine.

Examples

• Noun:
  • Hydra vulgaris is a well-studied freshwater genus within the class Hydrozoa.
  • The Portuguese man o' war, often mistaken for a jellyfish, is actually a colonial organism belonging to the class Hydrozoa.
  • The life cycle of a typical hydrozoan involves both a polyp and a medusa stage.

Advanced Usage

• "Hydrozoan" (adj/n): Pertaining to or a member of the class Hydrozoa.
  • The hydrozoan polyp colony was attached to the rock.
• In taxonomic context: Used to specify a rank within the phylum Cnidaria.
  • The order Siphonophora falls under the class Hydrozoa.

Variants and Related Words

• Hydrozoan (noun): An individual organism belonging to the class Hydrozoa.
  • The biologist studied the feeding habits of a hydrozoan.
• Hydroid (noun): Refers specifically to the polyp stage of a hydrozoan life cycle.
  • The hydroid stage is often colonial and sessile.

Synonyms

• Hydroid class (less common, contextual synonym in taxonomy).

Notes

• The term "class Hydrozoa" is a formal taxonomic classification. In general scientific writing, it is often not capitalized ("hydrozoans," "the hydrozoan class").
• While many jellyfishes are in the class Scyphozoa, some true jellyfishes, like those in the order Trachylina, are classified under Hydrozoa.

Noun

1. coelenterates typically having alternation of generations; hydroid phase is usually colonial giving rise to the medusoid phase by budding: hydras and jellyfishes