This slide shows the glochidium larva of a bivalve.  In most freshwater bivalves, eggs are fertilized internally by sperm entering with incurrent water flow.  After fertilization, the eggs develop within the gill tubes (which serve as temporary brood chambers) into the tiny bivalved glochidia larvae.  After being discharged, the larvae wait until they contact a passing fish and attach themselves to the gills or skin where they live as parasites for a few weeks before dropping off to begin living independent lives.  This larval "hitch hiking" is seen as an adaptation for dispersal among organisms whose powers of locomotion are otherwise very limited.