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This pair is generally uniramous, but is biramous in crabs and lobsters and remipedes. The pair attached to the second segment are called secondary antennae or simply antennae . The second antennae are plesiomorphically biramous, but many species later evolved uniramous pairs. [ 2 ]
The name Uniramia was temporarily rejected as a polyphyletic group, but when used now refers to the subphylum consisting of the insects + myriapods. Subphylum Uniramia is characterized by uniramous (single-branching) appendages, one pair of antennae and two pairs of mouthparts (single pairs of mandibles and maxillae). Their body forms and ...
The legs of insects and myriapods are uniramous. In crustaceans, the first antennae are uniramous, but the second antennae are biramous, as are the legs in most species. For a time, possession of uniramous limbs was believed to be a shared, derived character, so uniramous arthropods were grouped into a taxon called Uniramia. It is now believed ...
The first pair of head limbs were biramous antennae, with around 400 annulations on the longer ramus. The next four pairs were all relatively similar, with around 15 short podomeres, each with a stout spine. These appendages appear to have been uniramous, and what seems to be the fifth pair shows bunches of smaller, inward-facing spines.
The head carries three pairs of biramous limbs, alongside a many-segmented pair of uniramous antennae at least two-thirds as long as the entire body. No eyes, mouth or hypostome are known. The biramous limbs are all equivalent in structure, with large proximal segments on the endopods and more hook-like distal segments.
The three uniramous appendages had six segments (podomeres), each ending with a terminal claw, with the endopod of the fourth limb having a similar morphology. The trunk was divided up into 10 segments (tergites), all of approximately equal length and width, each of which were associated with pairs of biramous appendages.
Like other arthropods, crustaceans have an exoskeleton, which they moult to grow. They are distinguished from other groups of arthropods, such as insects, myriapods and chelicerates, by the possession of biramous (two-parted) limbs, and by their larval forms, such as the nauplius stage of branchiopods and copepods.
The undivided subovate-shaped dorsal shield covered the entire body, and wrapped around the front edge of the carapace, forming a doublure structure. The midline of the carapace exhibited thickening. There is a pair of slender uniramous segmented antennae, as well as ten pairs of biramous limbs.