Final answer:
Crustaceans are arthropods with a chitin-based exoskeleton, bilateral body symmetry, and primarily aquatic lifestyles. They are distinct from chelicerates like spiders and scorpions and have a range of specialized structures for digestion, waste removal, and circulation. Option a.
Step-by-step explanation:
Crustaceans are a diverse group of arthropods and include animals such as crabs, lobsters, shrimp, krill, and woodlice, all of which live primarily in aquatic environments, although some like woodlice have adapted to land.
Crustaceans are distinct from other arthropod subphyla, such as Chelicerata which includes animals like spiders and scorpions.
The exoskeleton of crustaceans is composed of chitin, sometimes infused with calcium carbonate for added strength, and serves multiple functions like protection and muscle attachment. Unlike animals with radial symmetry, like starfish, crustaceans have bilateral body symmetry.
As for the characteristics of crustaceans, they have an outer skeleton called an exoskeleton that they must shed in order to grow in a process called molting, a main body cavity with an expanded circulatory system,
a straight tube digestive system with a gastric mill for grinding food, and structures that function like kidneys for waste removal near the antennae.
Additionally, their nervous system consists of a brain in the form of ganglia. So option a.