Cnidarians have no organs and a one-way digestive system. They also have an incomplete digestive system and perform extracellular digestion. All cnidarians have tentacles or stingers used to immobilize their prey. They have two layers of tissue, and the inner layer lines the gastrovascular cavity. This inner layer is known as the gastrodermis. Between these two layers is a non-living substance that is similar to jelly, known as mesoglea. The tentacles take in larger food to the gastrovascular cavity, and cells in the cavity secrete enzymes to break down the food further so they can be absorbed by the cells. Diffusion allows the cells inside the cavity to exchange nutrients, waste, oxygen and carbon dioxide.
Platyhelminthes(also known as flatworms) have an incomplete digestive system, which means the digestive tract has one opening, and the digestion takes place in the gastrovascular cavity. A pharynx is present at the mouth for taking in food, followed by a highly branched intestine. These organisms mark the beginning of cephalization, which is the development in the head region of light-sensitive organs known as ocelli. There are three groups of Platyhelminthes that are parasitic and one group that is not parasitic. Parasitic Platyhelminthes live inside other animals and get their nourishment through the animal. Nonparasitic Platyhelminthes eat their own food. Parasitic worms are mostly anaerobes, meaning they don’t require oxygen for growth, and they have organs of adhesion such as spines, hooks and suckers. For parasitic worms, the nutrients can simply pass through their skin from the host animal.
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Nematoda(roundworms), like Platyhelminthes, these nematodes can have a parasitic or free-living lifestyle. Nematodes have a complete digestive system, which gives them the advantage of being able to continue to eat while their previous meal digests. They have a mouth at the start of their body, then a long tube with specialized parts and an anus at the end of their body. When food is ingested, it is moved along by peristalsis, and cells lining the tube absorb all the nutrients. Indigestible waste passes through the anus.
Mollusks also have a complete digestive system, that has a mouth, esophagus, stomach and anus. Their mouth has a structure similar to human teeth, known as the radula, which is used to scrape food or material into the mouth. The radula is supported by a structure known as the odontophore, and they are the only animal on earth with one of these. Once the food is scraped into the mouth, it is coated in mucus which helps to aid the cilia-like structures to move the food to the stomach. The mucus remains attached to the food all the way through the digestive system and is excreted through the anus.
Annelida's digestive system starts with a mouth and pharynx, followed by an esophagus, then an intestine and an anus. They have a complete digestive system and do not have teeth, but instead, they have a gizzard that smashes up their food so it can be easily digested. The worm eats food, and it is passed down to the esophagus to the crop, where it is stored. The food then passes to the gizzard where it is smashed up before it moves to the intestine. There are chemicals to break down the food further in the intestine, and the nutrients are absorbed through the wall of the intestine. The waste is excreted through the anus.
There are more than one million species of arthropods known to mankind, including insects, spiders, crabs, scorpions, millipedes and shrimp to name a few. The Arthropoda digestive system is complete and consists of a foregut (pharynx and esophagus), midgut (stomach) and hindgut (colon and anus). They have a diverse diet as a result of their variation of specialized appendages. Insects commonly have a crop in the foregut, and a gizzard to grind up their food. Crustaceans have an esophagus leading to the stomach where there is acid to help break down the food. They also have a gastric mill, similar to a gizzard, which grinds up the food with added enzymes. All food goes through the digestive system, and the nutrients are absorbed in the intestine and excreted through the anus.
Echinoderms have a simple digestive system, consisting of a mouth, esophagus, intestines, and anus. Commonly, the mouth is on the underside and the anus is on the upper side. Depending on the species, when food is ingested it is either passed by the pharynx to the intestines or to the esophagus. In species with an esophagus, they also have two stomachs, the cardiac stomach and the pyloric stomach. Digestive enzymes are secreted into the stomach and food begins digestion, then is passed to the second stomach where it continues to be digested. Then the food moves to the intestine where all nutrients are absorbed. Any remaining waste is passed through the anus.
Chordates are animals with a notochord. They are a diverse group of organisms and they eat by ingesting food rather than absorbing it. They have a complete digestive system, starting at the mouth and ending at the anus with various other organs along the way to help with digestion. The food begins at the mouth where it is ground up with teeth. Salivary glands help to start off the digestion process, then the food is moved down the esophagus by peristalsis. The food is now in the stomach, which contains chemicals and enzymes to break down the food more. Nutrients are absorbed in the intestine, and wastes are removed through the anus. Organs such as the liver, kidneys and pancreas help out with various parts of the digestion and removal of wastes. The liver makes bile and the pancreas produces enzymes to break down starches, sugars and fats.
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