Phylum Cnidaria

General Characteristics

• They are radially symmetrical; oral end terminates in 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

Movement

• The cnidarian body is capable of some kind of coordinated movement• Both the epidermis and the gastrodermis possess nerve cells arranged in a loose network - nerve net (plexus), which innervate primitively developed muscle fibers that extend from the epidermal and gastrodermal cells• Stimulus in one part will spread across the whole body via the network

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); some food is phagocytized by special cells and digestion occurs intracellularly• 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 chemosensation, the nematocyst is released from the cnidocyte and the coiled thread is everted• 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

Solitary Hydras• Freshwater hydras are found in ponds and streams occurring on the underside of vegetation• Most possess a pedal disc, mouth, hypostome surrounded by 6-10 tenetacles• Mouth opens to the gastrovascular cavity • The life cycle is simple: eggs and sperm are shed into the water and form fertilized eggs; planula is by passed with eggs hatching into young hydras • Asexual reproduction via budding

Class Hydrozoa cont.

Colonial Hydrozoans - e.g., Obelia

• Possess a skeleton of chiton that is secreted by the epidermis• All polyps in the colony are usually interconnected• Two different kinds of individuals that comprise the colony: feeding polyps or gastrozooids (C) and reproductive polyps or gonozooids (B)

Class Hydrozoa cont.

Life Cycle of Obelia• Gonozooids release free swimming medusae• Zygotes become planula larvae, which eventually settle to become polyp colonies

• The medusae of hydroids are smaller than those of jellyfishes (C. Scyphozoa)• Also, the margin of the bell projects inward forming a shelf-like velum

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

• Gametes develop in gastrodermis of gastric pouches; eggs and sperm are shed through mouth• Fertilized eggs develop into a planula larva; settles on substrate and develops into a polyp - scyphistoma• Scyphistoma produces a series of polyps by budding - strobila• The polyps undergo differentiation and are released from the strobila as free swimming ephyra• Ephyra matures into an adult jellyfish

Scyphozoan Life Cycle - Aurelia

Class Anthozoa• Exclusively marine; there is no medusa stage• At one or both ends of the mouth is a ciliated groove called the siphonoglyph; generates a water current and brings food to the gastrovascular cavity• Possess a well developed pharynx • The gastrovascular cavity is large and petitioned by septa or mesenteries; increase surface area for digestion or support• Edges of the septa usually have threadlike acontia threads, equipped with nematocysts and gland cells

Class Anthozoa cont.

• Solitary anthozoans include sea anemones• Most anthozoans are colonial (e.g. corals) and secrete external skeletons composed of calcium carbonate.• Corals obtain much of their energy from microscopic photosynthetic green algae (zooxanthellae) or dinoflagellates that live symbiotically inside the cells of the coral