Arthropods

Among the invertebrate visitors in the aquarium are members of the most successful and diverse phylum of them all— the arthropods. All the earth's creatures with jointed ("arthritis" inflames joints) legs (of which "tripods" have three) are arthropods. In the earth's history, arthropods got an earlier start than vertebrates, apparently even before the start of the Cambrian period, and ever since then they've had a more diverse success than the vertebrates, too.
 
The arthropods of the aquarium are almost all crustaceans (the minute springtails and water mites are exceptions). A characteristic of crustacean exoskeletons that sets them apart from the rest of the arthropods is that crustacea tend to deposit calcium in their hardened chitinous cuticle. This is a point to remember if you are trying to encourage Daphnia or shrimp, and your water is very soft. Because my New York water is so soft, I can maintain a culture of minute crustaceans in a nursery tank only by lining the bottom with crushed coral.
 
Crustacean arthropods are probably embodied for most fishkeepers in such live foods as brine shrimp or daphnia, or as "Amano" shrimp or ghost shrimp entrusted with algae control. But other minute arthropods, such as water mites (Hydrachnidia) or seed shrimp (Ostracods), are only seldom eaten by our fishes; so they rate here among the invertebrate visitors.
 
About chitin. A fundamental feature of arthropod hardiness is the chitin that forms their stiff yet flexible exoskeleton. Chitin is a tough, stringy polysaccharide molecule that can also be found in the cell walls of certain fungi. Chitin is insoluble in water and all but indigestible to most organisms— that's part of what makes it so essential to shrimps or insects. Only concentrated acid will dissolve it, and when the acid is diluted, the chitin will precipitate out again, unchanged. So chitin can be retrieved from shellfish processing, by using an acid bath. Chitin can find many industrial uses: it can be employed as a flocculating agent in wastewater management; it's a binder of dyes; it makes sizing for paper; it acts as an ion-exchange resin and as a thickener and stabilizer of food and medicine. More important for fishes, chitin provides essential roughage in the diet.