One of a class of amorphous nitrogenous principles, containing, as a rule, a small amount of sulphur; an albuminoid, as blood fibrin, casein of milk, etc. Proteids are present in nearly all animal fluids and make up the greater part of animal tissues and organs. They are also important constituents of vegetable tissues. See 2d Note under Food.

An order of aquatic amphibians having prominent external gills and four legs. It includes Proteus and Menobranchus (Necturus). Called also Proteoidea, and Proteida.