Arthropods generally lay eggs. If you mean how do the eggs get fertilized, the same way they usually do. One catch with big crustaceans is that they can only mate following a molt. Males of some crabs will literally keep a claw on a female who is about to molt until she' sheds her skin. Molted crabs are soft, which is where softshell crabs come from. However because their mouthparts are also soft, they don't eat for awhile following a molt. This can make them difficult to catch say blue crabs (Callinectes spp.)through conventional methods, which use bait. Many crustaceans carry the eggs until they hatch. Larval crustaceans are very small and are carried through the water column as plankton. Insects by contrast tend to have a mobile adult form and a relatively immobile larval form (think butterfly v. caterpillar). Crustaceans have a mobile larva and adults that don't travel great distances (or in the case of barnacles, not at all).

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I just have to say, GhostCrawler has quickly become my favorite developer... he's on the forums addressing a LOT of concerns, and he still finds time to be funny.