Usually gills for gas exchange. Fertilized eggs carried on female. Some hatchlings resemble adults, others have larval stages and successive molts. Barnacles are sessile and live in limestone cup they secrete. Suspension feeders with free-swimming larvae.
Isopods are tiny and mostly marine, but pillbugs and sowbugs are terrestrial.
Copepods are microscopic and most numerous part of plankton Decapods are lobster, crayfish, crab, shrimp. Carapace covers cephalothorax, of chitin and calcium salts. Antennae are chemoreceptors and tactile sense organs. Mandibles short, heavy, grinding and biting. First and 2nd maxillae are for feeding Maxillipeds chop up food and pass to mouth.