These elements form continuous tubules. There is a lot of range in the diversity in the animal kingdom. What an amazing mixture. Earthworm is segmented both externally and internally. They coordinate one set of circular muscles and another set of longitudinal muscles, which works against non-compressibly hydrostatic skeleton and can alter shape of each segment.
Earthworms alternately elongate and contract.
Their exoskeleton not only protects, but also provides points of attachment for appendage muscles. They are also segmented like annalids. MOst arthropods use either the shorter or rear appendages for moving in both sea and land. Placodurms have paired fins to aid swimming. Sharks have streamlined bodies, swift swimmer, but not very manuverable.