Medical Definition of mesozoan
: any of the animals that comprise the taxon Mesozoa, that includes small wormlike parasitic forms with an outer layer of somatic cells and an inner mass of reproductive cells, and that are often regarded as intermediate in organization between the protozoans and the metazoans although they may be degenerate descendents of more highly organized forms
Seen and Heard
What made you want to look up mesozoan? Please tell us where you read or heard it (including the quote, if possible).