Explain all the important characteristics of the given phyla: a. Platihelminthes b. Coelenterates c. Annelida
(a) Platyhelminthes are commonly known as flatworms or tapeworms. They are a group of soft-bodied invertebrate animals. As a matter of fact, there are around 20,000 species of these animals. A few of these live as parasites on humans and other animals. Furthermore, it is because of this parasitic nature that they do cause some amount of trouble for the host animal. A few species belonging to this phylum can be a major cause of certain diseases. For example, Schistosomiasis, or bilharzia or bilharziasis, is a disease caused by these parasitic flatworms. They belong to the family Schistosomatidae. The most distinguishing feature of these invertebrates is their flat body. As the body does not have any cavity, they are flat. The body is also not segmented and they do not have specialized systems. Around eighty percent of the flatworms are parasitic in nature, while a few free-form flatworms are also present. The free-living forms are scavengers or predators. The parasitic species feed on the tissues of the host organism in which they live. The animals in this phylum have a diverse range in size. Some are microscopic, while a few go up to two feet long. They are also hermaphrodites, which mean that both the sexes are present in the same organism. (b) These are typically invertebrate animals, which show a very simple level tissue organisation. They are aquatic animals and are mostly found in marine environments, attached to the rocks at the bottom of the ocean. A few species are also found in freshwater habitats. Coelenterates can be found solitarily or in colonies. You can find them sedentary or free swimming. Coelenterates can be called as the simplest animal group that has true tissues and have the characteristic coelenteron or the gastrovascular cavity. (c) Having over 17,000 species, Phylum Annelida is a large phylum. Annelids are also known as ringworms or segmented worms. They exist in various environments including marine waters, fresh waters and also in moist terrestrial areas. The size of the annelids can range from a few millimetres to an amazing three metres in length. The Australian earthworm measures around 3 metres. Furthermore some species from this phylum exhibit some unique shapes and brilliant colours. Annelids exhibit bilateral symmetry and are invertebrate organisms. They are coelomate and triploblastic. The body is segmented which is the most distinguishing feature of annelids.
