An introduction to hydroskeletons
Functional definition of a hydroskeleton
Requirements for a hydroskeleton
Generally, these types of animals have at least 2 pairs of musculature oriented in different directions, i.e. longitudinal and circular muscles.
Phyla with hydroskeletons
Nematoda (a pseudocoelomate phylum with a hydroskeleton)
Annelida (a coelomate phylum with a hydroskeleton)
Muscles among organisms with endo- and exoskeletons work as part of levers, providing the force.
Note how insect muscles also are connected across joints
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Other information: An introduction to hydroskeletons