The crustaceans (Crustacea) are a large group of arthropods (55,000 species), usually treated as a subphylum. They include various familiar animals, such as lobsters, crabs, shrimp and barnacles. They are variously found in marine and freshwater, with a few terrestrial members (such as woodlice).
Structure of Crustaceans
Crustaceans have 3 distinct body parts, head, thorax, and abdomen. They have two pairs of antennaes on the head with compound eyes. Also with three pairs of mouthpairs and a telson. Smaller crustaceans respire through body surface by diffusion and larger crustaceans respire by gills. Crustaceans typically have a thick carapace on dorsal part covering part of their body. Their appendages are typically biramous, including a second pair of antennae (but not the first).
The most important groups of crustaceans are barnacles, branchiopods, copepods and Malacostraca (crabs, lobsters, shrimps and krill). There are around 1,220 barnacle species, 1,000 branchiopods, 13,000 copepods, and 30,000 Malacostraca. All barnacles are marine and spend their adult lives attached to rocks. Barnacles are described as standing on their head feet. They are filter feeding organisms and filter feed by circulating water through its head feet.
The formal classification of crustaceans varies somewhat. Some authorities treat them as a class, in which case the groups listed in the taxonomic table are treated as subclasses. The groups listed here are generally recognized, but several of them are often treated as subclasses of a class Maxillopoda. In general, because of the large number of species, taxonomists have made extensive use of subordinate taxonomic categories (suborders, superfamilies and so forth), and the status of different groupings is frequently controversial; this can make taxonomic references hard to follow. Evolutionary relationships between the different groups are not entirely clear, making the exact definition of larger groups difficult. Good practice in describing cetaceans is therefore to include descriptions at several taxonomic levels, to ensure that readers can link the information to others' schemes.
Most crustaceans have separate sexes and are distinguished by apendages on the abdomen called swimmerets. The first and sometimes the second pair of swimmerets are larger comparatively to the female. Terrestial crabs mate seasonal and return to the sea to breed. Female crabs eggs are still retained by the females until hatch. When the eggs hatch, they hatch into free-swimming larvae.