These animals have a more complexly designed body than the Poriferans and Cnidarians. They are free-living or parasitic animals. 
The body design of Platyhelminthes:
  • Their body can be divided into two equal halves. Hence, they are bilaterally symmetrical
  • They have three layers of cells that make up differentiated tissues. Hence, they are known as triploblastic animals
  • Due to this, their body comprises of outside and inside body linings and organs for specific functions
  • They lack segmentation
  • They are acoelomates
  • As their body is flattened dorsoventrally (meaning from top to bottom), they are commonly known as flatworms
Clockwise from top left: Planaria, Fasciola hepatica (Liver fluke), Blood fluke, Taenia solium (tapeworm)
These are parasitic worms that cause diseases such as elephantiasis and filariasis. These are sexually dimorphic animals - female and male worms are distinct.
Body differentiation of nematodes:
  • These are bilaterally symmetrical, triploblastic animals
  • They have a cylindrical body rather than the flattened one, and hence they are commonly known as Roundworms or Pinworms
  • They have tissue-level of organisation
  • They lack organs
  • These are pseudocoelomates
Clockwise from top left: Ascaris lumbricoides (roundworm), Wuchereria bancrofti (filarial worm), Trichuris (whipworm), Enterobium vermicularis (pinworm)