Answer:
I believe they are called a population, tell me if I'm wrong.
That is a subduction or convergent boundary that can form volcanoes when two plates collide against each other.
These unsegmented worms have a full digestive system even when parasitic.
This process requires large volumes of water to pass through absorption surfaces to get enough oxygen into their bodies using their mouths and gills. These body parts work like a pump to keep water moving over the gas absorption surfaces of the gills.
