Crustacean

Crustaceans include all the animals of the phylum Arthropoda Crustacea; the word comes from the Latin crusta, which means shell. Crustaceans are a very diverse group of invertebrate animals which includes active animals such as the crabs, lobsters, shrimp, krill, copepods, amphipods, and more sessile creatures like barnacles. Arthropoda is the largest phylum of Animal Kingdom. It includes about 11,340,000 species in all habitats. This constitutes about 83% of all the known animal species on earth. Arthropoda includes spider, scorpions, prawns, crabs, millipedes, centipedes, and many other insects. Arthropoda is characterized by heteronomous metamerism, chitinous exoskeleton, and joined appendages. The evolutionary acquisition of these traits is known as arthropodization. In very small crustaceans, exchange of the respiratory gases occurs through the general body surface. Large aquatic arthropods respire through gills and book gills, whereas terrestrial forms respire through trachea and book lungs