Reconsideration of the nature of zooids in trepostomate Bryozoa defines them as physically connected and asexually replicated colony members that housed systems of organs necessary to perform vital functions for the colonies. Zooids known to contain organs in trepostomes are limited to autozooids, the requisite feeding and sexual units, and polymorphs, including macrozooids and two rare zooids of unknown function. Other colony structures are extrazooidal and remain outside zooidal boundaries throughout colony life. They include the commonly occurring mesopores, exilapores, and styles. This two-part morphologic division of colonies reveals two correlated functions. The essential autozooids dominated the growth patterns and physiology of trepostome colonies; the extrazooidal parts grew concurrently and passively to connect autozooids and to support and strengthen colonies.