Crabs are not arachnids; they are crustaceans. Arachnids are animals such as spiders. Arachnids, insects and crustaceans all belong to the family group arthropod. Arthropods are animals with exoskeletons, segmented bodies and jointed legs. They are the largest group of animals on Earth. Most crustaceans such as the crab are aquatic animals living in either marine or fresh water environments. While a few crustaceans such as terrestrial crabs and/or hermit crabs have adapted to life on land. Other crustaceans you may be familiar with include lobsters and barnacles.
Crustaceans can be recognized by their hard, external shell which protects their body. They are equipped with a head and an abdomen. Their heads include antennae which are a part of their sensory system. Their abdomen includes the heart, digestive system and reproductive system as well as appendages such as legs for crawling, swimming and climbing. Crabs love to climb. Many crustaceans have separate sexes and also reproduce sexually while a few others are hermaphrodites. Others have been known to change their sex during the course of their life.
Crustaceans such as the crab often have claws which help it crawl, eat, and defend itself against predators. Crabs usually pack quite a pinch. They burrow and can also run or walk sideways. These creatures can often live in extreme areas due to their ability to adapt to the changing environment. (An example of this would be the hermit crab which requires a temperature of 70+ degrees Fahrenheit and lots of moisture in order to live).