Eldonioids or velumbrellids are an extinct clade of disc-shaped cambroernids which lived in the early to middle Paleozoic (Cambrian to Devonian). Depending on the researcher, the clade is either considered to either be the class Eldonioidea (containing only the order Velumbrellida) or be the order Eldonida. The lifestyle of eldonioids is still an unresolved question; some authors reconstruct eldonioids as free-floating planktonic predators similar to jellyfish, while others argue that they were passive detritivores, embedded within the seabed for much of their life.

The terms "eldonioid" and "eldoniid" have been used somewhat informally and interchangeably, but technically refer to members of the class Eldonioidea and the family Eldoniidae, respectively. When using the order Eldonida, the adjectival form is "eldonid".

Eldonioids are characterized by their "medusoid" (jellyfish-shaped) bodies, with the form of a shallow dome opening below to an offset mouth supplemented by filamentous tentacles. Internally, they have a distinctive C-shaped cavity encompassing the gut, as well as hollow radial (radiating) structures arranged around a central ring canal. Most eldonioids are soft-bodied and can only be preserved in lagerstätten, but a few species may have hosted mineralized deposits. Historically, the affinities of eldonioids was enigmatic; recently, they been assessed as cambroernid deuterostomes.

The basic eldonioid body plan is oriented around two circular discs sealed together with the viscera in between. The ventral (lower) disc is concave from below while the dorsal (upper) is convex from above, providing a dome-like profile to the body. The rim of the body is smooth, though the margin may furl slightly inwards under the ventral disc. Sediment infillings or differences in preservation allow for hollow internal structures to be differentiated from solid sheets of tissue within the body.

The ventral disc acts as a platform for numerous elongated internal structures arranged in a radiating pattern. These narrow structures, known as "radial sacs" (also "radial lobes" or "internal lobes"), were most likely hollow cavities. They are among the most visible features in most fossils. The sacs are divided from each other by thin ligamentous walls, described as "septa" or "mesenteries". In most eldonioid species, each radial sac bifurcates (divides into two parts) near the rim of the disc. The radial sacs thin towards the center of the disc, but do not converge at a single point. Instead, they intersect with a small ring-shaped cavity or canal in the central part of the ventral disc. The radial sacs and ring cavity were likely filled with fluids, akin to a hydrostatic skeleton or (less likely) an echinoderm-like water vascular system.

The dorsal disc is usually more rigid and resistant to distortion than the ventral disc. In several taxa, the dorsal disk is ornamented by one or more irregular concentric (ring-shaped) wrinkles, which may be growth lines or artefacts of compression. According to some interpretations, radial ridges, wrinkles, or grooves also ornament part of the dorsal disc in some rotadiscids. Paropsonemids have even more complex dorsal discs, combining radial and concentric ornamentation at a fine level of detail. In Eldonia and Stellostomites, the dorsal disc appears to bear radial internal structures, mimicking the radial sacs above the ventral disc. The dorsal disc has been interpreted as almost completely solid, with the radiating structures identified as very thin tubular canals. This is opposite to what is seen in the ventral disc, which has proportionally larger radiating cavities and slender solid septa.

In most eldonioids, the center of the body is solid, but in other species, there is a rigid central cavity which tapers towards the ventral or dorsal disc. All eldonioids bear a thick, easily-recognizable coiled sac which rings around the middle of the body. The coiled sac is a horseshoe-shaped cavity that curves dextrally (clockwise) from the mouth to the anus. It sheaths the alimentary canal (gut), which is thinner and more difficult to discern in most fossils. In the best-preserved eldonioids, the gut can be divided into three regions: the esophagus or pharynx (front), stomach (middle), and intestine (rear). The stomach is usually the broadest part of the gut, and the portion of the coiled sac surrounding it is stained a dark color. The mouth and anus are positioned close to each other and open through the ventral disc.

A small number of circumoral feeding tentacles project out from near the mouth. Their basic form is shrub-shaped, with a pair of main shafts that split away from each other and divide further into smaller filaments. Some authors referred to the circumoral tentacles as a "lophophore", a term used for the hollow filamentous tentacles of lophophorates such as brachiopods and bryozoans. In Rotadiscus, a pair of tightly coiled spiral structures connect to the base of the tentacles. These structures covered a hole called the coelomopore, seen in living ambulacrarians. Rotadiscus helps to establish homology between the coelomopore and additional pharyngeal openings present in tunicates and cephalochordates, as well as part of the vertebrate pituitary gland. Moreover, it suggests that the tentacles develop from outgrowths of the coelom (body cavity), more akin to ambulacrarians rather than lophophorates. Further similarities include detachment of the tentacles from the mouth, and their tapering structure.