Phylum Cnidaria Introduction
Phylum Cnidaria: formerly part of the Coelenterate Phylum: which included the ctenophores
Bauplan Blue indicates new feature (synapomorphy) • Two embryonic germ layers • Ectoderm • Epidermis • Nervous Tissue • Endoderm • Gastrodermis • Mesogloea is the nonliving gelatinous material in between the two tissue layers
Bauplan • Sessile, sedentary or pelagic
Bauplan • Ring of tentacles
Bauplan • Radial symmetry as adults • Polyps are biradial
Bauplan • Noncentralized nerve net with radially distributed sense organs • Neurons serve sensory and motor systems.
Epithelial layer of a cnidarian showing the epitheliomuscular cells with the myonemes. Sensory nerve cells can also be seen between the cells
Nerve Net • Specialized cells for communication = Neurons! • Fused connection around body like a net • No cephalization; no brain-like structure; • No nerve cord; no ganglia • Senses environment • Rhopalium containing sensory receptors • Bidirectional Signal • Can send and receive signal with same cell • Responds with graded contraction
Nerve Net and Sensory Abilities • More complex cnidaria (sea anemones and jellyfish) may have two nerve nets. • Slow conducting network • Fine fibers end at neuromuscular junction • Coordinates slower and more delicate movements • Fast conducting network • Large bipolar cells beneath epithelium • Bigger fibers, faster conduction of impulse • Enables major responses, particularly in times of danger.
Hydra Nerve Net neurite neuron
The greater the frequency of stimulation, the greater the surface area affected
Rhopalia • Sensory organs (rhopalia) of medusae • Statocysts: balance organs • Orientation in water • Ocelli: light receptors • Important for diurnal migration • Important for species with endosymbiotic algae • Touch receptors • Chemoreceptors
Bauplan 2 Body Forms
Bauplan • Life history • Alternation of generations • Alternate polyp and medusa forms • Cnidocytes • Stinging cells armed with nematocysts used for defense and prey capture
Cnidocytes • Stinging cells • Defense • Prey capture • Structure - several different kinds • Cnidocil = trigger • Made of specialized cilia • Cnidocyte contains nematocyst with its stinging thread and barbs
Cnidae • Three types of cnidae are found in the cnidocytes of cnidarians • Nematocysts (in nematocytes) – toxic, found in all Cnidarians • Protein or phenol-based toxin • Spirocysts (in spirocytes) - sticky • Muco- or glycoprotein to form a long sticky thread which adheres to the surface of the prey rather than penetratingit • Also used to attach the animal if it’s dislodged from its perch only in Anthozoa
Cnidae • Ptychocysts - only found in the tube-building anemones, • used to "spin" the fiberglass-like tube in which these animals live.
Cnidocyte before discharge. Cnidocil and operculum with the coiled tube still contained in the cnidocyte. cnidocyte
Cnidocytes • Operation • Touching cnidocil or chemical triggering of chemoreceptors stimulates firing • Actual mechanism unknown - based on osmotic pressure Cnidocyte with nematocyst Cnidocyte with discharged nematocyst
Mechanism of Firing • Three hypotheses have been proposed to explain the mechanism of firing: • The capsule becomes permeable to water just before firing, and the rapid influx of water to the capsule results in the explosive release of the thread; • The manufacture of the capsule leads to a high internal pressure and that pressure drives the thread out of the capsule and into the prey • The cnidocyte contracts violently to cause the thread to be discharged by squeezing the capsule.
Mechanism of Firing • Hypothesis #3 is almost certainly incorrect • Because cnidae "stolen" from cnidarians for self-defense by other animals (called kleptocnidae ) require only the capsule itself, and not the living cell (none of these animals that steal cnidae from cnidarians can ever produce more once the capsule is fired - they must consume new prey to regain functional cnidae). • Recent work with high-speed microcinema-tography suggests that both the first two hypotheses may play a role in firing the cnidae.
Mechanism of Firing • Discharge can occur simply in response to touch, but most require both a mechanical and a chemical stimulus to discharge. • An individual cnida can only ever be fired once. • The cell that manufacturers the capsule can replace the organelle once discharged, but they are all single-use structures
Bauplan • Gastrovascular cavity • Incomplete gut • Only one opening: mouth = anus
Bauplan • Hydrostatic skeleton • Uses water instead of solid materials • Contractiletissues/locomotion
Hydrostatic Skeletons • A skeleton • Is a solid or fluid system that permits muscles to be stretched back to their original length following a contraction • Muscles can only contract on their own • They cannot stretch themselves • Allows for antagonistic actions of muscles • May or may not have protective and supportive functions as well
Hydrostatic Skeletons • Hydrostatic skeleton plan • Cavity containing an incompressible fluid (such as water) that transmits pressure changes in all directions • Volume of fluid remains constant • Cavity is surrounded by flexible outer body membrane that permits the outer body wall to change change • Keep track of how the hydrostatic skeleton is used in the rest of the phyla we cover
Hydrostatic skeleton and antagonistic muscles of a cnidarian. Note how body length and girth change as circular and longitudinalmuscles contract.