Final answer:
An arachnid differs from a crustacean by having four pairs of legs, specialized appendages, internal breathing systems for terrestrial life, and often being carnivorous and venomous, compared to crustaceans' biramous appendages, gills, and typically aquatic lifestyle.
Step-by-step explanation:
An arachnid differs from a crustacean because arachnids have four pairs of legs, specialized appendages like chelicerae and pedipalps, a body organized into the cephalothorax and abdomen, internal breathing systems adapted for land such as trachea or book lungs, and they are mostly carnivorous. In contrast, crustaceans typically possess two pairs of antennae, biramous appendages, gills for underwater gas exchange, and are often found in aquatic environments.
Furthermore, several arachnids are venomous and may be parasitic, while crustaceans have a wide diversity of about 47,000 described species which include predominant aquatic organisms like shrimp, crabs, and lobsters, as well as a few terrestrial species.