The tubules have pores that lead out of the body. Water and ammonia leaves through these pores. Aerobic respiration produces carbon dioxide as a waste product. This is released from the planarian by diffusion. To carry out aerobic respiration, the planarian must get oxygen to the cells of its body.
Oxygen enters the planarian by diffusion.
The special features of the Phylum Platyhelminthes includes the protonephridia. These structures can be considered primitive kidneys. They are unique to the phylum. Planarians are placed in the Class Turbellaria, and these were discussed extensively above. The animals generally have a sucker around the mouth and one on the ventral surface.