Two nice sites on invertebrates that live in our aquarium:
The first www.invertebrati.info is an italian site about shrimp, crabs, snails and other invertebrates can populate our aquariums.
It contains many in-depth articles (very good that on shrimp's anatomy), cards for the recognition of animals, and some magnificent pictures galleries.
Second one Crusta 10 is a German site (also in English), mainly dedicated to crabs, shrimps and crayfish.
It has a more scientific approach and less amateur than the previous. Lists many articles and publications on these animals and even here there are cards for identification / classification and photo galleries.
The two sites are complementary with no overlap and are very definitely two links to keep handy for fans.