A crustacean is an arthropod belonging to the subphylum Crustacea. They are a diverse group, with over 67,000 known species, found in a wide range of habitats, from the deep sea to freshwater lakes and rivers, and even on land.

Here are some key characteristics of crustaceans:

* Exoskeleton: Crustaceans have a hard, external shell made of chitin, called an exoskeleton. This provides protection and support.

* Segmented Body: Their bodies are segmented, with distinct head, thorax, and abdomen regions.

* Jointed Appendages: They have jointed appendages, including legs, antennae, and mouthparts.

* Two Pairs of Antennae: Crustaceans have two pairs of antennae, which are sensory organs used to detect food, mates, and danger.

* Gills: Most crustaceans breathe through gills, which are feathery organs that absorb oxygen from the water.

* Molting: To grow, crustaceans must shed their exoskeletons in a process called molting.

* Variety of Feeding Habits: Crustaceans have a variety of feeding habits, including filter feeding, scavenging, and predation.

Some common examples of crustaceans include:

* Crabs: They have a broad, flattened carapace and 10 legs, including a pair of large claws.

* Lobsters: They have a long, segmented body, two large claws, and 10 legs.

* Shrimp: They have a curved body, a pair of small claws, and 10 legs.

* Barnacles: They are sessile crustaceans that attach themselves to rocks, ships, and other surfaces.

* Woodlice: They are terrestrial crustaceans that breathe through modified gills and live in damp, dark environments.

Crustaceans play important roles in their ecosystems, serving as food sources for larger animals and helping to decompose organic matter. They are also a significant source of food for humans.