Platyhelminthes
The Platyhelminthes, also known as flatworms are a phylum of unsegmented, bilaterian, soft bodied invertebrates. What makes them unique to other bilaterians is that they have no body cavity and no specialized respiratory of circulatory organs. As a result, it is harder for them to absorb oxygen through diffusion.
Excretory System
The Platyhelminthes excretory system consists of fine protonephridial tubules embedded within the mesoderm. Its function is really quite simple. This excretory systems main function appears to be to remove excess water from the body of the organism, although waste metabolic products may also be flushed out at the same time. Within the Digenean trematodes typically the protonephridia drain into a pair of longitudinal ducts, opening into a posterior bladder which is connected to the exterior via an excretory pore.
Examples