Annelida | Animalia | Diversity in Living Organisms

7.5.5 ANNELIDA Branched Eyes gastrovascular Annelid animals are also bilaterally symmetrical and triploblastic, but in addition they have a true body cavity. This allows true organs to be packaged in the body structure. There is, thus, extensive organ differentiation. This differentiation occurs in a segmental fashion, with the segments lined up one after cavity Scolex Sucker Acetabulum Neck Pharynx Mouth and anus the other from head to tail. These animals are found in a variety of habitats- fresh water, Planareia Liverfluke Таре worm marine water as well as land. Earthworms and leeches are familiar examples (see Fig. 7.16). Fig. 7.14: Platyhelminthes Tentacle 7.5.4 NEMATODA Palp The nematode body is also bilaterally symmetrical and triploblastic. However, the body is cylindrical rather than flattened. There are tissues, but no real organs, although a sort of body cavity or a pseudo- coelom, is present. These are very familiar as parasitic worms causing diseases, such as the worms causing elephantiasis (filarial worms) or the worms in the intestines (roundworm or pinworms). Some examples are shown in Fig. 7.15. Parapodia Genital рapillae Anus Parapodia Nereis Earthworm Leech -Female Fig. 7.16: Annelida 7.5.6 ARTHROPODA Male This is probably the largest group of animals. These animals are bilaterally symmetrical and segmented. There is an open circulatory system, and so the blood does not flow in well- defined blood vessels. The coelomic cavity is blood-filled. They have jointed legs (the word 'arthropod' means 'jointed legs'). Some familiar examples are prawns, butterflies, houseflies, spiders, scorpions and crabs (see Fig. 7.17). Ascaris Wuchereria Fig. 7.15: Nematodes (Aschelminthes)