As nouns the difference between sponges and protozoan

is that sponges is plural of lang=en while protozoan is any of the diverse group of eukaryotes, of the phylum Protozoa, that are primarily unicellular, existing singly or aggregating into colonies, are usually nonphotosynthetic, and are often classified further into phyla according to their capacity for and means of motility, as by pseudopods, flagella, or cilia.

As a verb sponges

is third-person singular of sponge.

As an adjective protozoan is

of, pertaining to, or characteristic of a protozoan.