In this less specialised method of reproduction, usually used by crabs and some shrimp species. The fry undergo several stages of development, during which time they don't resemble the adults at all, before they become young adults.
When born from their tiny eggs, they are of the form nauplius, which have 3 pairs of limbs, including the antennae and maxillaries. They simply drift through the water feeding on infusoria, using the antennae to swim.
Soon more body segments develop, and as more segments appear they become a higher form of plankton.
With the first development of a new segment they become metanauplius, then, as they mature further, protozoea and finally forms of zeoa (which itself can have up to 5 stages of development). The zeoa are closest to, and ultimately become, the adult crustacean.
Under natural conditions these tiny larvae are carried, in their various stages of development, through the currents. They remain in the planktonic state for a considerable amount of time, and feed on increasing sizes of plankton as they develope.
The fry losses in an aquarium and indeed in nature are very high, and a lot become food, for this reason the females can produce hundreds of thousands of eggs, unlike the few hundred produced by species using the specialised method.
For the aquarist, the breeding of these decapods is fairly easy. Needing only to perform regular water changes and keep the parents well conditioned.
The female will carry the eggs in her pleopods for about 3 weeks, then the tiny fry will hatch in their primitive form.
With crabs, the eggs are carried on the shell across land, and are deposited in either brackish, or even sea water.
Raising the fry, is extremely difficult if not impossible for most. They will need very specific tiny foods, like protazoa and rotifers, that can only be obtained from specialists (marine specialists are best). The newly hatched fry can also be so small they are almost, if not totally, invisible to the eye.