Mostly marine or aquatic. No appendages on abdomen. Thoracic leaf shaped appendages for gas exchange. This group is extremely diverse in habitat use. Rhizocephalans are internal parasites of crabs.
Copepods can also be external parasites of fish.
Head, thorax, and abdominal segments present. Head and thorax often fused together and covered with a carapace. Appendages present on abdomen. Compound eyes usually present. In contrast to the crustaceans, these organisms all have uniramous appendages.