Free living, commensal or parasitic forms.
Tissue level grade of organisation.
Triploblastic: Body derived from 3 embryonic germ layers.
Bilaterally symmetrical with definite polarity of anterior and posterior ends.
Dorsoventrally flattened, with a well defined ventral surface bearing mouth and gonopore.
Body unsegmented.
Acoelomate — without any body cavity. Spaces between various organs is filled with special mesodermal tissue.
Adhesive structures like hooks, spines, suckers and adhesive secretions.
Muscular system of mesodermal origin. Longitudinal, circular and oblique muscle layers beneath epidermis.
Digestive system branched and incomplete without Anus. Altogether absent in Acoela and Cestoda.
Degenerated skeletal, respiratory and circulatory.
Excretory System has lateral canals and protonephridia (flame cells).
Nervous System primitive and ladder like.
Sense organs simple. Eye spots or photoreceptors in free living forms.
Mostly hermaphrodite with complex reproductive system. Well developed gonads, gonoducts and accessory organs. Eggs are mostly devoid of yolk.
Fertilisation is internal.