Phylum Cnidaria Formerly: The Coelenterates General Information Habitat Mostly marine A few freshwater (Hydra; freshwater jellyfish Crasspedacusta can be found in Illinois) Life history Alternation of generations Polyp (usually asexual) Medusa (sexual) Movement Sessile or pelagic Depends on life history stage Diversity >10,000 species known 1
Bauplan (Blue indicates new feature or synapomorphy) Two embryonic germ layers Ectoderm Endoderm (Gastrodermis) Mesogloea is the nonliving gelatinous material in between the two tissue layers Tissue grade have true tissues Radial symmetry Sessile, sedentary or pelagic Ring of tentacles Noncentralized nerve net with radially distributed sense organs 2
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 2 Muscle filaments = myonemes Bauplan Life history alternation of generations: alternate polyp and medusa forms Cnidocytes (stinging cells) armed with nematocysts used for defense and prey capture (Sometimes called nematocytes) Gastrovascular cavity Incomplete gut has only one opening Hydrostatic skeleton Contractile tissues/locomotion Planula larva 3
More on Cnidarian Bauplan Hydrostatic Skeleton Nerve Net Sensory Organs 4
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 longitudinal muscles contract. 5
Bauplan Sessile, sedentary or pelagic Bauplan Ring of tentacles 6
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= Gastrovascular Cavity Epithelial layer of a cnidarian showing the epitheliomuscular cells with the myonemes. Sensory nerve cells can also be seen between the cells 8
Hydra Locate: gastrovascular cavity, gastrodermis, epidermis, mesoglea cs ls Hydra Locate: epitheliomuscular cells, gastrovascular cavity, gastrodermis, cnidocytes, mesogloea cs ls 9
Hydra cs ls Hydra ls Hydra 10
Hydra cs 11
Siphonoglyphs Bauplan Radial symmetry as adults Polyps are biradial 12
Bauplan Noncentralized nerve net with radially distributed sense organs Neurons serve sensory and motor systems. Nerve Net and Sensory Abilities No central nervous system Nerve net is composed of neurons in contact with one another and with epitheliomuscular cells. Cells and nerve fibers Neurons = cells Neurites = fibers extending from cells Cells may have many neurites So impulse can go in many directions at once 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 13
Hydra Nerve Net neurite neuron Epithelial layer of a cnidarian showing the epitheliomuscular cells with the myonemes. Sensory nerve cells can also be seen between the cells Myonemes 14
Cnidarian Nervous Systems Septum 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. The greater the frequency of stimulation, the greater the surface area affected 15
Nerve net of Velella - Smaller nerves arrear to be synaptic while larger nerves may be syncitial as a result of small nerve cells fusing during development. Rhopalia (-ium) 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 (thigmoreceptors) Chemoreceptors 16
Rhopalium of Aurelia 17
Stinging cells Defense Prey capture Cnidocytes Structure - several different kinds Cnidocil = trigger Made of specialized cilia Cnidocyte contains nematocyst with its stinging thread and barbs Cnidocyte before discharge. Cnidocil and operculum with the coiled tube still contained in the cnidocyte. cnidocyte 18
only in Anthozoa 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 Three types of cnidae found in the cnidocytes of cnidarians 1. Nematocysts (in nematocytes) toxic, found in all Cnidarians Protein or phenol-based toxin 2. Spirocysts (in spirocytes) - sticky Muco- or glycoprotein to form a long sticky thread which adheres to the surface of the prey rather than penetrating it Also used to attach the animal if it s dislodged from its perch 3. Ptychocysts - only found in the tube-building anemones, used to "spin" the fiberglass-like tube in which these animals live. Types of Cnidae Cnidae are the means by which sea anemones and other cnidarians capture prey and defend against predators. Nematocysts have double-walled capsules and can be stinging or tangling depending on the type of cnidarian. Spirocyst is a type of adhesive organelle found only in sea anemones. Spirocysts have single-wall capsules, discharge branched or single threads, are nonpenetrating and non-venomous, and contribute mainly to the adhesive stickiness of a sea-anemone tentacle to both animate and inanimate objects. 19
Mechanism of Firing Three hypotheses have been proposed to explain the mechanism of firing: 1. 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; 2. 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 3. The cnidocyte contracts violently to cause the thread to be discharged by squeezing the capsule. 20
Mechanism of Firing Hypothesis #3 is almost certainly incorrect Because cnidae "stolen" from cnidarians for selfdefense 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 microcinematography 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 21
Discharged cnidocyte showing the everted stinging tube cnidocyte Figure 06_01 22
Hinged operculum 23
Schematic showing the 3 main elements causing prey (or gelatincoated nylon bead) to stick to an anemone's tentacle. Of these, nematocyst-thread adhesion and mucous stickiness are the main attachment forces. Of course, the adhesive strength of capsule attachment to the tentacle may ultimately govern whether a prey will rip free. Cnidae are shown with attendant companion (supporting) cells. Potency of toxins A single nematocyst has been shown to suffice in paralyzing a small Drosophila larva The most deadly cnidocytes (to humans, at least) are found on the body of a box jellyfish. One member of this family, the sea wasp, Chironex fleckeri, is claimed to be the most venomous marine animal known. Neurotoxin attacks nervous system Second toxin attacks heart Third destroys skin movie 24
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Swimming using coronal muscle contractions Creeping and summersaulting in Hydra 27
Planula larva - Thought to be important in the evolution of metazoans 28
Planula Larva Develops from fertilized egg/zygote Ciliated and free swimming Dispersal stage 29