The members of this phylum are commonly called flat worms because of their dorsoventrally flattened body. Though they do not have body cavity, it is only the mouth that opens into the alimentary canal. Flat worms are bilaterally symmetrical and triploblastic animals with three body layers. They are hermaphrodites with complicated reproductive system. Examples are liver flukes and tapeworms.
Nematoda have cylindrical body with pointed ends hence, their common name-roundworms. There is the body cavity called coelum. They are bilaterally symmetrical and triploblastic animals with the alimentary system having two openings mouth and anus, A thick but elastic cuticle covers the entire body surface. They have separate sexes. Many of them are serious parasite of man, animals and plants. Example are, Ascaris, Hookworm, Threadworm, and Rootknot nematodes.
This is the largest group kingdom comprising of metamerically segment, triploblastic, coelomate and bilaterally symmetrical animals with segment jointed paired appendages and chitinous exoskeleton. They distinct head region (pronounced cephalization). The phylum is divided into four main classes:
Crustaceans: These are arthropods with two pairs of antennae and a pair of compound eyes on movable stalks. The head and the thorax are fused and covered by the carapace. They have up to five pairs of legs. Many of them have very hard chalky exoskeleton (e.g. woodlouse, waterleaf). They are found mainly in aquatic environment.
Arachnida: members of this class have two-body division (cephalothorax and abdomen). Many simple eyes (not compound eye) are present on the cephalothorax are four pairs of jointed legs for walking. Examples are, Spider, Scorpion, and tick.
Insects: The body is divided into three parts, head, thorax, and abdomen. The head bears one pair of antennae, a pair of compound eyes and three pairs of mouth parts. The thorax have three segment each with a pair of walking legs. Usually, wings are present on the second and third thoracic segments. Insects are the only invertebrate that can fly. Examples are, cockroach, cricket, grasshopper, mosquito, butterfly, bee, and termite.
Myriapoda: These are arthropods with numerous walking legs attached to each of the several body segments in pairs. These are of antennae on the head. Examples are, centipede and millipede. The centipedes have flattened body with few segment each bearing a pair of legs. This is a pair of poison claws, whose bite paralyses its preys like worms, insects and spiders. The millipede have cylindrical body with two pairs of legs per body segment. On the head is a pair of antennae.
Picture Source - Wikipidia (CCO)
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