Antennae or antenna-like structures

a) 2 pairs

Crustaceans have two pairs of sensory limbs on their heads: the antennules and the antennae. Both may have two branches. In Ostracoda both structures are used in feeding. In Isopoda the antennules are minute and difficult to see, but they are readily apparent in Amphipoda.

b) triflagellate, bead-like

Pauropods have bifurcate (sometimes seeming trifurcate because of a sensory process) antennae with bead-like flagella and no eyes.

c) 1 pair linear

Antennae are preoral sensory limbs that originate on the head in front of the eyes (when present) and above the mouth. Some insects and hexapods have reduced or seemingly absent antennae (or they may be broken off), and may have well developed labial or maxillary palps that can be confused with antennae (but antennae will also be present). Some snout mites have elbowed palps that are rather antenna-like.

d) 1 pair elbowed (geniculate)

Ants, other hymenopterans, and some other Insecta have elbowed (i.e., geniculate) antennae with a long basal scape and a distal flagellum that is made up of a few to many segments.

Misinterpretations allowed: Some mites have geniculate palps that may look like antennae. (You can choose to disable "Allow misinterpretations" in the Preferences (gear icon) options.)

e) 1 pair ring-like

Velvet worms have a series of constrictions in their body, antennae, and legs (lobopods), that give them the appearance of being constructed of a series of rings.

f) elbowed antenna-like palps

Snout mites (Prostigmata, Bdelloidea) have elbowed palps that look rather antenna-like, and their six-legged larvae could be confused with a scale crawler or similar minute insect. However, the forward projecting mouthparts and the lack of segmentation should help to distinguish these mites from other small arthropods.

g) antenna-like legs

Amblypygids and uropygids (Thelyponida) have attenuate and whip-like legs I with the tarsi subdivided into numerous pseudosegments.

h) biflagellate

Crustaceans often have two branches or flagellae on each antenna (as well as having two pairs of antennae).

i) antennae absent or seemingly so