A plexus (from the Latin for "braid") is a branching network of vessels or nerves. The vessels may be blood vessels (veins, capillaries) or lymphatic vessels. The nerves are typically axons outside the central nervous system.
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Although many medical words ending in -us that came to English from Latin have the plural suffix -i (and the plural form plexi indeed does exist in Latin), English does not use the -us/-i pattern for this particular term; the standard plural form in English is plexuses.
Plexuses
In invertebrates
The plexus is the characteristic form of nervous system in the coelenterates and persists with modifications in the flatworms. The nerves of the radially symmetric echinoderms also take this form, where a plexus underlies the ectoderm of these animals and deeper in the body other nerve cells form plexuses of limited extent.