Which physical traits do scientists use to classify malacostracans?

Scientists classify malacostracans based on a fixed body plan consisting of a head, a thorax, and an abdomen. Key physical traits used for classification include:1

• Segmentation: The head typically comprises five segments, the thorax eight, and the abdomen usually six (rarely seven) unfused segments.1
• Appendages: Classification considers the structure of antennae, mouthparts, and thoracic legs. Malacostracans generally possess two pairs of antennae and three pairs of chewing mouthparts. Thoracic legs are typically biramous (two-branched), though they may be modified into walking limbs or pincerlike structures depending on the species.1
• Carapace: The presence, size, and fusion of the carapace—a head shield—are critical diagnostic features. It may cover the thorax, be reduced, or be absent entirely.12
• Abdominal Features: Scientists examine the presence of ventral, biramous limbs (pleopods) used for swimming or reproduction, as well as the terminal segment, which typically bears a pair of uropods and a median plate called the telson.12
• Eyes: The position of the eyes, whether stalked or sessile (fixed to the head), is also a distinguishing characteristic.12