What is Subphylum Crustacea and Subphylum Uniramia?

The Arthropoda are usually grouped into four subgroupings called subphyla (singular-subphylum). One of these four subphyla - the Trilobita, which lived in the oceans millions of years ago in the Paleozoic era, is extinct. The remaining three subphyla are separated into three subphyla based upon types of mouthparts and types of appendages.

The Subphylum Chelicerata includes those arthropods that do not have jaws. All have 6 pairs of appendages, four of which are walking legs, and two pairs that are modified into mouthparts. The first pair of appendages form fang-like pincers called chelicerae. The second pair behind the chelicerae is usually a set of feelers that help in feeding, and are called pedipalps.

However, another major distinguishing feature is used to separate the two subphyla. All of the appendages of crustaceans are branched into two parts. These appendages are referred to as biramous. The legs of insects, centipedes, and millipedes, on the other hand, are uniramous, or unbranched. The name of the subphylum is taken from this descriptive feature.

The Subphylum Chelicerata is grouped into three Classes. The Class Arachnida is very large, made up of eleven different Orders. Arachnida Orders include the terrestrial spiders (Order Araneae), ticks and mites (Order Acari), scorpions (Order Scorpiones), and the daddy longlegs (Order Opiliones).

The distinctive horseshoe crabs form their own class, Class Merostomata, and are thought to belong to a group that can be traced back 220 million years in the fossil record. The Class Pycnogonida include the sea spiders, which are small and inconspicuous, yet common in marine habitats.