This metamerism extends from external segmentation to the serial repetition of parts internally as well.
New segments arise posteriorly.
And the blastopore in these worms becomes the mouth. They are protostomes and their development is determinate. The septa that divide the worm into segments are made up of the peritoneum of adjacent segments. These septa divide the coelom into individual hydrostatic units. When longitudinal muscle contracts, a segment shortens and the coelomic fluid exerts pressure outward laterally.