Phylum Platyhelminthes

Phylum Platyhelminthes

Platyhelminths (flatworms) are the simplest animals to show true bilateral symmetry and three embryonic germ layers (triploblastic organisation), yet they remain acoelomate — they have no body cavity between the body wall and the internal organs, which are instead packed in a solid mass of mesodermal tissue (parenchyma). Their bodies are characteristically flattened dorsoventrally, which increases the surface area available for gas exchange, since the group lacks dedicated respiratory organs.

Excretory system — flatworms possess a distinctive excretory system built around flame cells, specialised ciliated cells found at the blind end of branching excretory tubules throughout the body. Each flame cell has a tuft of cilia inside a small chamber that beats continuously (resembling a flickering flame under the microscope), driving fluid and waste material along the tubules toward excretory pores on the body surface. This flame-cell system is the characteristic excretory apparatus of the phylum, distinguishing it from the excretory organs (nephridia) of later, coelomate phyla such as Annelida. …