In phylum Cnidarian, which contains the hydras, jellies, corals and sea anemones most are motile while a select few remain sessile. For the members of phylum Platyhelminthes all can move. Members of this phyla include tapeworms, planarians, and the flukes. These creatures are either free-living (planarians) meaning they live in marine waters, fresh waters and soil or they are parasitic (tapeworms/flukes) meaning they live within a host. They platys have a flattened body with a opening for a mouth and a separate opening for the anus. Out of the mouth is a large muscular tube called the pharynx which is an adaptation used to pump food into its digestive cavity. Just like the cnidarians, this phyla has bilateral symmetry giving it cephalization or a head. Within the head of the platy species is a "simple brain" or ganglia which are a group of nerve cells that control the nervous system. Attached to the ganglia are two long nerve cords that stretch down the sides of the body. The shorter nerve cords run across the body in a horizontal fashion. On the anterior part of the body lay eyespots or a group of cells used to detect the changes in amount of light in an environment. In addition to having eyespots most flatworms have specialized cells which detect external stimuli such as the chemicals found in food or which way water is flowing. Flatworm's nervous system is much more advanced than those of cnidarian which only contain ocelli and a primitive nerve net not even advanced enough to be called a brain. The flatworms also developed flame cells which act like a primitive kidney that flush out metabolic waste, whereas cnidarians relied heavily on diffusion of H2O particles to remove its waste. Diffusion still plays a huge part for members of Platyhelminthes as they rely on it for respiration just like the cnidarians. The gastrovascular cavity continues to make a strong presences in flatworms, although they have developed other features like the pharynx to ease digestion, making things run smoother. The development of the third germ layer also makes these animals further developed than the previous phylum. However the non-existence of a coelom and the specific jobs for digestive organs leave plenty of room for further development and advancement. Unlike flatworms, the cnidarians use their stinger cells filled with poison to paralyze or kill its prey before taking it through its mouth and to the gastrovascualr cavity for further digestion. In terms of reproduction, flatworms have the choice of using fission (splitting into two new identical worms) which is identified as asexual reproduction or remaining hermaphroditic meaning they have both the male and female parts, therefore using sexual reproduction. As you know, the cnidarians may apply asexual reproduction through budding or shoot motile gametes into the water through sexual reproduction. To end things off, both the flatworms and cnidarians can agree upon circulation as they both do not have specialized organs to circulate. Diffusion is what they rely primarily on.
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