Phylum Cnidaria General Characteristics They are radially symmetrical

Phylum Cnidaria

General Characteristics • They are radially symmetrical; end with a mouth surrounded by tentacles. • They have 2 tissue layers • Outer layer of cells - the epidermis • Inner gastrodermis, which lines the gut cavity or gastrovascular cavity (gastrodermis secretes digestive juices into the gastrovascular cavity) • In between these tissue layers is a noncellular jelly-like material called mesoglea

Cnidarian Body Plans Polyp form • Tubular body, with the mouth directed upward. • Around the mouth are a whorl of feeding tentacles. • Only have a small amount of mesoglea • Sessile Medusa form • Bell-shaped or umbrella shaped body, with the mouth is directed downward. • Small tentacles, directed downward. • Possess a large amount of mesoglea • Motile, move by weak contractions of body

Nutrition • Cnidarians are carnivores with hydras and corals consuming plankton and some of the sea anenomes consuming small fishes • They use they tentacles to capture prey and direct it toward the mouth so that it can be digested in the gastrovascular cavity via secretions from gland cells (extracellular digestion) • The gastrovascular cavity exists as 1 opening for food intake and the elimination of waste • There is no system of internal transport, gas exchange or excretion; all these processes take place via diffusion

Stinging Organelles • Prey capture is enhanced by use of specialized stinging cells called cnidocytes located in the outer epidermis. • Each cnidocyte has a modified cilium cnidocil, and is armed with a stinging structure called a nematocyst. • The undischarged nematocyst is composed of a long coiled thread • When triggered to release, either by touch or chemical response, the nematocyst is released from the cnidocyte and the coiled thread is ejected • Some nematocysts function to entangle the prey; others harpoon prey and inject a paralyzing toxin

Reproduction • One of the most amazing adaptations is the ability of some cnidarians to regenerate lost parts or even a complete body • Asexual reproduction is common with new individuals being produced by budding • Sea anenomes engage in a form of asexual reproduction called pedal laceration • Cnidariand are dioecious • Fertilization is external, with the zygote becoming a elongated, ciliated, radially symmetrical larva - planula larva Planula larva

Cnidarian Taxonomy

Class Hydrozoa • Includes the solitary freshwater hydra; most are colonial and marine • Typical life cycle includes both asexual polyps and sexual medusa stages; however, freshwater hydras and some marine hydroids do not have a medusa stage

Class Hydrozoa cont. Other Hydrozoans Portuguese man-of-war: Single gas-filled float with tentacles Tentacles house the polyps and modified medusae of the colony

Class Scyphozoa Jellyfish • The medusae are large and contain massive amounts of mesoglea • The differ from the hydrozoan medusa in that the lack a velum • Possess four gastric pouches lined with nematocysts; these are connected with the mouth an the gastrovascular system

Class Anthozoa • Exclusively marine; there is no medusa stage • • Possess a well developed pharynx •

Class Cubozoa • polyps and medusae stages, but medusae dominate with polyp stage reduced. • polyp stage develops directly into medusa. • includes box jellies and sea wasps.