Class XI, BIOLOGY, "Phylum Platyhelminthes (Flat worms)"

Phylum Platyhelminthes (Flat worms)

MAIN CHARACTERS
HABIT AND HABITAT
Animals are mostly Parasitic in habitat and found in other higher animals. But some animals are also free living.
NATURE
They are triploblastic in nature i.e. body is composed of three germinal layers, viz, ectoderm, mesoderm and endoderm
EXTERNAL FEATURES

• Their bodies are compressed dorsoventrally and shows bilateral symmetry.
• Body shape generally worm like but vary from moderately elongated flattened to long flat ribbons and leaf like.
• The flat worms are small to moderate in size varying from microscopic to as long as up to 10-15 m.
• Majority of animals are white or colourless, some derive colour from ingested food.
• Anterior end of body is differentiated into head.
• Ventral surface bearing mouth and genital pores.
• Presence of great variety of adhesive parts e.g. suckers.
• Body is covered by cuticle or by ciliated epidermis.
• Hard part consist of cuticle, spines, thorns or hooks etc.

INTERNAL FEATURES

• Endo and Exo skeleton are completely absent, hence the body is generally soft.
• Acoelomate i.e. true coelom is absent.
• Body space between various organs is filled with MESENCHYME.
• Digestive system is poorly developed or absent.
• Respiratory and Circulatory systems are absent.
• Excretory system consists of Protonephridia or flame cells.
• Nervous system is primitive. The main nervous system consists of a pair of cerebral ganglia or brain and 1-3 pairs of longitudinal nerve cords, connected to each other by transverse commissures.

REPRODUCTION

• Platyhelmenthes are hermaphrodite i.e. male and female sex organs are present in same individual.
• In majority of forms eggs are devoid of Yolk but provided with special yolk cells.
• Cross fertilization as well as self fertilization is present.
• Life cycle may be simple or complicated involved one or more hosts.

EXAMPLES
Planaria, Liver flukes, Schistosoma and Taenia Solium etc.