Which body systems are absent in flatworms?

Flatworms (phylum Platyhelminthes) are characterized by a relatively simple body organization. They lack several specialized systems found in more complex animals, specifically:

• Respiratory system: Flatworms do not have specialized organs for breathing; instead, gases diffuse directly across their body wall.12
• Circulatory system: They lack an independent vascular or circulatory system, relying on diffusion and their flattened body shape to transport nutrients and gases.13
• Skeletal system: Flatworms do not possess a skeletal system.1
• Body cavity: They are acoelomates, meaning they lack a true body cavity (coelom).14

Additionally, while many flatworms possess a digestive system, it is important to note that Cestoda (tapeworms) have completely dispensed with a digestive tract, absorbing nutrients directly from their hosts across their body wall. Most other flatworms lack an anus, with undigested residue typically passing back out through the mouth.53