Phylum Cnidaria

Common Name: Corals, Anemones, and Jellyfish

Etymology: "Nettle", from the Greek knide for nettle, in reference to the stinging cells (nematocysts) characteristic of the group.

Representative examples:

Click images to enlarge.
The sea pen, Ptilosarcus	Hydra, an aberrant freshwater anthomedusan (shown budding)	The medusa of Polyorchis, an anthomedusan	Pelagia, a large semaeostoman medusan	Gonionemus. A hydrozoan medusa

Symmetry: Radial Tissue Layers: 2 (ectoderm, endoderm and acellular mesoglea, derived from ectoderm) Coelom: None Digestive Tract: Incomplete, simple gastrovascualr cavity Special Features: Cnidocytes = "nettle cells", prey-capture and defensive cells found on animals of the phylum Cnidaria. Cnidocytes have unique organelles in them called cnidae that are used to capture prey. Cnidae are composed of a coiled thread-like structure attached to the bulb-shaped cell body. The externally-oriented side of a cnidocyte cell also has a hair-like trigger on it. When the trigger is activated the cell expels the thread-like structure. Some cnidae puncture the skin or exoskeleton of an organism and inject a paralyzing toxin, these are called nematocysts. Others stick to the surface of the prey item and yet others coil themselves around cellular projections on the prey. Depending on the species there may be one or more types of cnidae on the organism. click to enlarge

Additional Information:

Cnidarians are diploblastic eumetazoans (multi-cellular animals with two true tissue layers, they lack mesoderm, instead having a gel-like acellular mesoglea) and are distinct from the other groups studied in lab in displaying radial symmetry, placing them on an evolutionary lineage (the Radiata) separate from the other groups we will focus on (Bilateria). Cnidarians are often colonial (the corals are the best examples here) and many species have a dimorphic life cycle with one stage a sessile polyp form and the other a motile medusa form. Subgroups within the Cnidaria are distinguished by which of the two morphs (if either) dominates the life cycle. Note that medusae and polyps are essentially the same body plan, simply inverted in orientation.

Cnidarians have an incomplete digestive tract with a single opening to their simple gastrovascular cavity. They are not cephalized and have a decentralized "nerve net" nervous system with a few poorly-developed sensory structures, appropriate for a sessile or slow-moving, radially-symmetric organism to sense and react to stimuli from any direction. They do not have discrete resiratory, circulatory, or excretory organs and rely heavily on diffusion for these functions. Although they lack true muscles (which normally arise from mesoderm), cnidarians do have simple contractile tissues which provide for movement and act as part of their hydrostatic skeletons. Digestion is extracellular with enzymes being secreted into the gastovascular cavity. The general simplicity of the cnidarian body is shown by the fact that an entire cnidarian organism has fewer cell types than single organs in many other metazoan groups.

Cnidarians are principally marine predators although a few species inhabit fresh water and some forms augment their nutrition by housing symbiotic photosynthetic organisms in their tissues.

Link:

• Cnidaria (Earthlife.net)
• Plain, Simple, Primitive? Not the Jellyfish

More images:

Click images to enlarge and see full caption and photo credit. Images © Deep Sea Images
Anemone with Sea Fleas (amphipods)	Magnificent Anemone	Anemone - Heteractis magnifica	Golden Zoanthids - Parazoanthus swiftii