These are animals that are found in freshwater and marine water. They are bilaterally symmetrical and triploblastic animals with a true body cavity. Hence, they have definite organs with an extensive organ differentiation -organ system level of organisation. The body of these organisms is differentiated in a segmented fashion from head to tail.
They also have a closed circulatory system—their locomotory structures known as parapodia for their locomotion.
Clockwise from left to right: Earthworms, leeches, Neries
The phylum Arthropoda consists of the largest group of animals. These animals are bilaterally symmetrical and triploblastic with a true body cavity (coelomates). They have an organ-system level of classification as they have definite organs.
Other characteristics:
  • They have an open circulatory system as their blood does not flow in blood vessels.s
  • Their coelomic cavity is blood-filled.
  • They have jointed legs. Hence, they are known as arthropods as the term arthropod means jointed legs.
Clocwise from top left: Prawn, crab, housefly, spider, butterfly, scorpion, millipede