Biology Sikshya

    Gr., platy=flat; helminthes=worms

• They may be present in aquatic habitat or in moist terrestrial habitat.

• They may be free living or parasitic.

• Triploblastic animals i.e., body derived from 3 germ layer (ectoderm, mesoderm and endoderm).

• Body is bilaterally symmetrical.

• Body is dorsoventrally flattened. It is tape or leaf like structure. Epidermis is soft, syncytial and in some cases it is ciliated or may be covered by cuticle. Body covering is thick.

• Body segmentation is absent.

• Tissue-organ or  organ grade of body organization is seen.

• Coelom is absent.

• Locomotion takes place by cilia and by the contraction and expansion of the muscles i.e., gliding movement.

• They may have a gastrovascular cavity.

• Parasitic forms may be monogenic or digenic.

• They may be ectoparasites or endoparasites. They have suckers to attach themself to the host body or the particular food material. Hooks and spines are also seen in some forms.

• Digestion may be extracellular or intracellular. Digestive system is incomplete type.

• Blood vascular system is absent. Transportation takes place by diffusion.

• ​​Respiration takes place by normal body surface.​​

• Excretion takes place by the action of specialized cells called flame cells or protonephridia. These cells absorb excretory products, which are excreted out of the body through nephridiopores. Flame cells also helps in osmoregulation.

• Nervous system is poorly developed, and consists of a brain ganglion and a pair of connected nerve cords.

• Sense organ is also poorly developed and an eye-spot is found.

• Asexual reproduction occurs by regeneration. Sexual reproduction occurs by releasing sperms and ova.

• Development may be direct or indirect.

• Fertilization is internal.

• These animals are hermaphrodite animals, but cross fertilization is the rule.