22 November

Cnidaria

General features

• Extracellular digestion--bring food into gastrovascular cavity where it is subjected to breakdown by enzymes produced by cells lining the cavity
• Intracellular digestion--particles are phagocytocized by cells lining cavity. In some groups, these cells have flagella that beat to mix up contents.
• Incomplete digestive tract

Foodgetting

Use tentacles which are covered with nematocysts either single or in batteries.

Nematocysts are produced by cnidoblasts. The nematocyst is actually a cell product, as once produced, they have no nucleus. Different types (stinging, sticky, entangling). Are fired as a structure, the cnidocil , acts as a trigger. Work by everting the "nema". This occurs because of changes in membrane permeability brought about by the trigger event. Trigger event may be both mechanical and chemical.

Feeding of Physalia and Hydra --capture food with tentacles. In Physalia, items are surrounded by gastrozoids which engulf the food item (small fish) and secrete enzymes into the space surrounding fish. Particles are then taken into gastrozooids and phagocytosed. Hydra captures zooplankters with tentacles, opens gullet and stuffs items into gastrovascular cavity wither extra and intracellular digestive occurs.

corals and their symbionts --although corals can feed much like any other cnidarian, much of their nutrition comes from the symbionts. Symbionts also play a strong role associated with the secretion of calcium carbonate

• zooanthellae
• zoochlorellae

Use of nematocysts by other organisms-some species of nudibranches eat hydrozoans, but are able to extract nematocysts without firing them and move them into their cerratia.

Ctenophores --structurally much like the cnidaria, except lack nematocysts and have eight row of ciliated plates. Some have two tentacles. Have colloblasts.

voraceous planktonic feeders--as group moves through patch of plankton, they pretty much "vacuum-up" whatever is in their way.

Platyhelminthes

Incomplete digestive tract

Tricladia--planaria --evertible pharynx. Find food by chemosensory means. Food is usually in the state of decay. Use pharynx to pump food into their digestive tract. Few wastes as food is mostly in liquid form. Nitrogenous breakdown products are handled by excretory system

Polyclads--marine worms.