Hydrozoa
-Hydrozoans are both marine and freshwater (only freshwater class).
-They do display generation alterations (polyp to medusa).
-They do display generation alterations (polyp to medusa).
*Distinguishing Characteristics:
(1) They only have nemocytes in the epidermis.
(2) Gametes are epidermal and released to the outside of the body rather than into the gastrovascular cavity.
(3) The mesoglea is largely acellular.
(2) Gametes are epidermal and released to the outside of the body rather than into the gastrovascular cavity.
(3) The mesoglea is largely acellular.
*Polyps (Obelia):
-Most hydrozoans have colonial polyps in which individuals may be specialized for feeding, producing medusae by budding, or defending the colony.
-In Obelia (type of cnidarian), the planula larvae develops into a feeding polyp called a gastrozoid. Gastroids have tntacles, feeds on microscopic organisms, and secretes a skeleton of protein and chitin.
-A gonozoid (a.k.a gonagium) is a reproductive polyp that produces medusae by buddying.
-A gonozoid (a.k.a gonagium) is a reproductive polyp that produces medusae by buddying.
*Medusae (Gnionemus):
-In the hydrozoan Gnionemus, the medusa is the predominate stage.
-The medusa project inward to form a shelf-like lip called a velum which is only seen in hydrozoan medusae.
-Medusae have a connection of nerve cells called a nerve ring that encircles the medusae's margin used for coordinating swimming movements.
-The Gnioemus is dioecious and sheds gametes into the seawater. The planula larvae attatches to the substrate (layer that underlies something) and eventually becomes a polyp.
-The medusa project inward to form a shelf-like lip called a velum which is only seen in hydrozoan medusae.
-Medusae have a connection of nerve cells called a nerve ring that encircles the medusae's margin used for coordinating swimming movements.
-The Gnioemus is dioecious and sheds gametes into the seawater. The planula larvae attatches to the substrate (layer that underlies something) and eventually becomes a polyp.
*Hydra:
-The freshwater Hydra is a hydrozoan that lacks the medusa form and reproduced both asexually from budding from the side of the polyp and sexually through the forming of testes and ovaries through the mitosis of certain epidermal cells (interstitial cells).