Classes of Cnidaria • Class Hydrozoa (e.g., Hydra, Obelia) • Class Scyphozoa (e.g., Aurelia) • Class Cubozoa (e.g., cube jellies) • Class Anthozoa (anemones and corals) • Subclass Hexacorallia (sea anemones, hard corals) • Subclass Octocarallia (sof corals)
General Characteristics • Entirely aquatic; mostly marine, but with a few freshwater species; • may be solitary or colonial • Metazoan, with true tissues. • Diploblastic – two germ layers • outer cell layer known as ectoderm (epidermis), • inner cell layer known as endoderm (=gastrodermis), • with a non-cellular mesoglea between.
General Characteristics • The internal body space (gastrovascular cavity) lined with gastrodermis • has a mouth, but no anus (incomplete digestive tract) • Possess cnidocytes with special cell organelles - nematocysts, used for defense and offense
General Characteristics cont. • Phylum exhibits polymorphism • Metagenesis- alternation of generation • Polyp (often sessile) and the medusa (free swimming). • The polyp stage reproduce asexually, while the medusa stage usually reproduces sexually • Reproduction is asexual by budding and/or sexual, producing a ciliated planula larva
Figure 13.02 Two general body forms
Class Hydrozoa • solitary or colonial • alternation of asexual polyps and sexual medusae
Life Cycle of Hydra SEXUAL REPRODUCTION ASEXUAL REPRODUCTION
Hydra with Bud Ovary Bud
Hydra with Gonads Testes
Gastrovascular cavity Hydra c.s. Epidermis Mesoglea Gastrodermis
Figure 13.09 Obelia Life Cycle
Obelia - Medusa Gonad Tentacle Gastrovascular cavity
Obelia – Hydroid colony Gastrozoids = Hydranth Gonozoids = Gonangium
Obelia – Hydroid Colony Gonozooid Hydrozooid
Physalia –Portuguese Man-O-War - a colony, not an individual It is not a jellyfish!!
Class ScyphozoaTrue Jellyfishes • polyp stage reduced or absent • bell-shaped medusae • margin of bell with eight notches which are provided with sense organs • all marine
Class – AnthozoaSea Anemones and Corals • all species occur as polyps only, no medusae; • gastrovascular cavity partitioned by at least eight septa with nematocysts • Solitary or colonial • all marine.