Classification Of Animals

Piatyhelminthes (eg, Planaria, liver fluke and tapeworm) possess the simplest tubular excretory systems called protonephridia. A protonephridium is a network of closed tubules that terminate in cellular units called flame cells or flame bulbs. The flame cells open into excretory ducts. Excretory materials diffuse from the surrounding tissues into the flame cells. Vibrations of the cilia cause these materials to remove in the excretory ducts.

In biradial symmetry, the body can be divided into two similar halves by one or two vertical planes only, eg, sea anemones. The animals which show radial and biradial symmetry have oral and aboral sides. The oral side has mouth, whereas the aboral side is one which is opposite to oral side.