How Do Cnidarians Use Their Tentacles Answers.com
The phylum Cnidaria (pronounced "nih DARE ee uh") includes soft-bodied stinging animals such as corals, bounding main anemones, and jellyfish (Fig. 3.23 A). The phylum's name is derived from the Greek root word cnid- pregnant nettle, a stinging plant. Cnidarians are found in many aquatic environments. Sea anemones are widely distributed, from cold arctic waters to the equator, from shallow tide pools to the bottom of the deep body of water. Jellyfish float near the surface of the open oceans and in some tropical freshwater lakes. Corals are found primarily in shallow tropical waters, simply a few grow in deep cold ocean waters. Minor anemone-like cnidarians like Hydra sp. are likewise found in freshwater lakes and streams. Cnidarians range in size from tiny animals no bigger than a pinhead to graceful giants with trailing tentacles several meters long.
Some animals that look similar to cnidarians are really non part of the aforementioned phylum. An instance of this is a type of jelly called a ctenophore (Fig. 3.23 B). Ctenophores were removed from the phylum Cnidaria and placed in a new phylum called Ctenophora (pronounced ti-NOF-or-uh). Although both ctenophores and cnidarians take like bodies with thin tissue layers enclosing a middle layer of jellylike textile, scientists now group them separately. These rummage rows, called ctenes (ctene meaning comb) is how the ctenophores go their common proper name of comb jellies.
In the phylum Porifera we saw a body formed of aggregated cells with no system into tissue layers or organs. Cnidarians accept a slightly more organized body program, and accept tissues, only no organs. Near cnidarians have ii tissue layers. The outer layer, the ectoderm, has cells that aid in capturing food and cells that secrete mucus. The inner layer, the endoderm, has cells that produce digestive enzymes and break up food particles. The jellylike material betwixt the 2 layers is called the mesoglea. All of these torso layers surround a cardinal cavity called the gastrovascular cavity, which extends into the hollow tentacles (Fig. 3.24). Figure 3.24 demonstrates the anatomy of the main cnidarian forms.
The body plans cnidarians more often than not take radial symmetry (Fig. 3.25 A). Considering the tentacles of corals, jellyfish, and sea anemones take this radial construction, they can sting and capture food coming from whatever direction.
Many cnidarians take two main structural forms during their life cycles, a polyp class and a medusa form. The polyp form has a body shaped like a hollow cylinder or a pocketbook that opens and closes at the height (Fig. 3.25 A). Tentacles form a band around a pocket-sized oral fissure at the summit of the bag. The mouth leads to a primal torso cavity, the gastrovascular cavity (Fig. 3.24 B). Polyps attach to hard surfaces with their mouths up. Because they are sessile organisms, they tin can just capture food that touches their tentacles. Their mesoglea layer is very sparse. Corals and sea anemones are polyps. Most of these animals are modest, but a few body of water anemones tin can grow every bit large as 1 meter in diameter. The second structural form that cnidarians have is chosen the medusa class. Medusa bodies are shaped like an umbrella with the oral fissure and tentacles hanging down in the water. The mouth leads upward into the gastrovascular cavity. Medusae (plural; the singular form is medusa) are non sessile, but rather are motile, meaning that they swim freely in the ocean (Fig. three.25 C). Their mesoglea is thick and makes upwardly most of their bulk. Jellyfish are medusae. Medusae come in many sizes ranging from small 2.5-centimeter-long box jellies to the lion's mane jellyfish, which has an umbrella over ii grand beyond. In many ways polyps and medusae are really the same basic body plan, except each is upside down compared to the other. Some cnidarians go through both a polyp and medusa phase in their life cycle. All the same, i or the other is the ascendant phase in different species. Figure 3.25 demonstrates some examples of torso plans showing radial symmetry.
Cnidarians take a unique feature: stinging cells called cnidocytes (NID-uh-sites). Each cnidocyte prison cell has a long, coiled, tubular harpoon-similar structure, chosen a nematocyst (Greek root word nema meaning thread; Greek root word cyst meaning bag). The unfired nematocyst is inverted into itself, much like a sock bunched up and turned inside out. When the nematocyst senses nutrient either through touch or chemoreception, it fires outward, injecting venom through its tube into the prey (Fig. 3.26). Each nematocyst can fire just once, just new cnidocytes abound to replace used ones. The structure of cnidocytes is specific to different species of cnidarians.
All cnidarians are carnivorous predators. Jellyfish capture pocket-sized drifting animals with their stinging cnidocyte-filled tentacles. Even the sessile coral polyps and bounding main anemones are predators ready to sting prey, grasp it in their tentacles, and button it into their mouth. The potency of the stinging venom varies among species. Some cnidarian venoms have little event on humans. Others are extremely toxic. The venom of the Portuguese man-of-war (Physalia physalis) is potent enough to inflict a painful sting, even after information technology is done upward on the beach.
Unlike sponges, which have skeletal structures fabricated of spongin or spicules, ocean anemones and jellyfish have no skeletal structure to support their soft tissues. For support, they fill the gastrovascular cavity with water and close the mouth tight, putting the h2o under pressure as in a balloon filled with water. The water pressure supports the soft tissues. This feature is chosen a hydrostatic skeleton (Fig. iii.27). If the sea anemone opens its oral cavity or contracts its body wall difficult, the h2o flows out and the body collapses. It takes several minutes to pump water back into the crenel. Coral polyps also have a hydrostatic skeleton, only they are often sitting in a difficult skeleton fabricated of the mineral limestone (calcium carbonate or CaCOiii). Coral reefs are the aggregated limestone skeletons of many coral polyps.
Cnidarians lack organs. This means that they do non have respiratory or circulatory systems. Similar the cells in sponges, the cells in cnidarians go oxygen straight from the water surrounding them. Nutrients from digested nutrient pass through the liquid betwixt the cells to nourish all parts of the body, and wastes laissez passer out by the aforementioned route. Cnidarians have a very simple nervous system consisting of cells with long, thin fibers that reply to mechanical or chemical stimuli. The fibers connect, forming a network called a nerve net (Fig. iii.28). The nerves transport impulses to muscle cells, which respond by contracting. Despite its lack of complication, the nerve net does let cnidarians to respond to their environment.
Cnidarians practise take a more sophisticated sensory biology than sponges. The power to respond to a stimulus of touch on or pressure is called mechanoreception. When something touches the surface of the sea anemone, the nerve cells send impulses to the musculus cells in the body wall, the musculus cells contract, and the anemone moves. Chemoreception is the ability to answer to chemical stimuli. Chemoreception includes sense of taste and smell, two ways to observe chemicals. Chemoreception is crucial to finding and testing foods, detecting harmful substances, and, in some organisms, selecting and alluring mates and finding suitable places to live. Cnidarians rely on chemoreception for these things, too. The ability to reply to changes in calorie-free intensity is called photoreception. Most cnidarians accept the ability to sense changes in light and dark. Box jellies have eyes that are able to form images, making them the most derived cnidarians in terms of sensory biology. Finally, nigh jellyfish too have a sensory structure called a statocyst that is denser than water. The gravitational pull on the statocyst helps ocean going jellies tell which mode is down.
To respond to stimuli, cnidarians utilize a rudimentary muscular system consisting of musculus cells lying in bands up and down the torso wall and in a circle around the oral fissure crenel (Fig. 3.27). The trunk shortens when the vertical bands contract. If muscles on only one side contract, the trunk bends in that management. The mouth closes when the circular musculus contracts.
Many jellyfish are supported by an umbrella shaped structure that is equanimous of a modified layer of mesoglea. When a band of muscles contracts, a jet of water is forced out from nether the umbrella, moving the jellyfish forwards. When the muscles relax, the stiff mesoglea springs back to its original shape, and the umbrella opens again (Fig. 3.29). Alternate muscle contraction and relaxation creates pulsating movements that propel the jellyfish through the water. Yet, jellyfish are such poor swimmers that they are considered plankton. Plankton are aquatic organisms that cannot swim against a electric current.
Bank check out the video for an introduction to jellyfish movement and function.
Cnidarians reproduce both sexually and asexually. Some species tin produce both eggs and sperm in the aforementioned organism. These organisms are called simultaneous hermaphrodites and release gametes into the body of water in egg-sperm bundles. Some species are also either male or female and produce either eggs or sperm. Fertilization (the uniting of egg and sperm) tin happen externally in the water column, only can also happen internally. Many coral species reproduce externally in a procedure called circulate spawning (Fig. 3.thirty B). These species tend to take synchronous spawning events in which all individuals in the colony or area release their gametes at the same time. This is oftentimes triggered by ecology cues similar full moons, temperature, or chemical signals from other individuals. Circulate spawning increases the likelihood of sperm and egg from the aforementioned species meeting and for genetic mixing to take place. In other cnidarians the male releases sperm into the water, but fertilization happens inside the body when sperm from a male colony enters the female and fertilizes eggs internally. This type of sexual reproduction is called brooding, resulting in the release of a fully formed larva (Fig. 3.30 C).
Following fertilization in broadcast spawning cnidarians, the new organism grows into a larva that swims by means of cilia—modest hair-like structures that move information technology along by beating back and forth. Because larvae cannot easily swim against currents, they are classified as plankton, organisms that drift. The larval stage is important in dispersing sessile species like coral. Larvae tin stay afloat for a long time, globe-trotting hundreds of miles from the parent, or they tin settle within hours after fertilization. An anemone or coral larva remains in the water column until it can find a suitable habitat, attach to a difficult surface, and grow into a sessile adult (Fig. three.thirty).
Cnidarians can also reproduce asexually, by budding or fragmentation (Fig. 3.30 D, Due east). If many attached buds are produced, they can class a large colony. This is the mode of reproduction for which reef-building corals are famous. They can form such large colonies that they alter the structure of the ocean floor. Cnidarians tin also supercede lost or damaged parts by regeneration. Damaged or lost tentacles can often grow back. A small-scale chunk of discrete tissue may even regenerate into an entire new organism, every bit in the freshwater anemone Hydra sp. Sea anemones can besides regenerate lost parts.
How Do Cnidarians Use Their Tentacles Answers.com,
Source: https://manoa.hawaii.edu/exploringourfluidearth/biological/invertebrates/phylum-cnidaria
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