Biramous is a term used for branched arthropod appendages. These are most commonly branched into a gill and leg with a common root at a body segment. Each leg/gill structure will be paired with a second biramous structure on the other side of the body. Biramous appendages are best known from trilobites where all the legs -- both on the head and the thorax -- are parts of biramous structures with a gill branching off above the leg. The converse of biramous is uniramous or -- very rarely -- monoramous. Uniramous structures are unbranched. Uniramous structures are also found in arthropods. Examples of uniramous structures are trilobite antennae and the legs of chelicerates. Subphyla and Classes Subphylum Trilobitomorpha Trilobita - Trilobites (extinct) Subphylum Chelicerata Arachnida - Spiders, Scorpions, etc. ... Orders Agnostida Redlichiida Corynexochida Lichida Nektaspida? Phacopida Proetida Asaphida Harpetida Ptychopariida Trilobites are extinct arthropods in the class Trilobita. ... The limb of an arthropod is said to be uniramous when it has only one branch. ... Subphyla and Classes Subphylum Trilobitomorpha Trilobita - Trilobites (extinct) Subphylum Chelicerata Arachnida - Spiders, Scorpions, etc. ... Orders Agnostida Redlichiida Corynexochida Lichida Nektaspida? Phacopida Proetida Asaphida Harpetida Ptychopariida Trilobites are extinct arthropods in the class Trilobita. ... Antennae (singular antenna), are the paired appendages connecting to the first (and in crustaceans also to the second) segment of the head of the members of all subphyla of the arthropods except Chelicerata. ... Classes Arachnida - spiders, scorpions, etc. ...