There are two sets of muscles that help annelids move. One expands, and the other contracts the body. Annelids alternate the use of these muscles to achieve locomotion.
The annelid is a phylum that contains about 15,000 different species. This phylum contains all of the segmented worms and includes three different classes. These classes are Polychaeta, which contains marine worms, Oligochaeta, which contains terrestrial worms, and Hirudinea, which contains leeches.
Annelids have body walls with longitudinal and circular muscle layers that also help with their movement. The circular muscles surround each segmented part and the longitudinal muscles run along the length of the worm. When the annelid contracts the circular muscles, the segments elongate and thin. The longitudinal muscles then bring the segments back to their original shape, and the annelid is able to move forward. Annelids have metameric bodies that are divided into segments. In an adult, the locomotor activity is greater because they have more segments and this increases their flexibility. The segments are divided by septa and have a fluid-filled cavity inside known as the coelom. The head and rear of the annelid is not segmented. The coelom creates a hydrostatic skeleton and gives the segmented parts rigidity.