Fertilisation in bryozoans is internal. Sperm from one colony are captured by a recipient colony's tentacles, to which they first adhere then move downwards towards a duct (intertentacular organ) or, more commonly, a pore that allows entry into the body cavity. Fertilised eggs may be incubated in a special sac within the zooid, within a modified zooid (gonozooid or brood chamber), or, in the majority of species, in a hood-like ovicell distal to the maternal orifice. Ancestrally, the ovicell is derived from modified spines produced at the proximal end of the next zooid in the series, an evolutionary transformation that is preserved in Cretaceous forebears.
The Bryozoa is the only major phylum not found fossil in the Cambrian. Remarkably, at least six orders of bryozoans are represented in the next period, in the Early Ordovician. Bryozoans since that time have left an exceptional fossil record, mostly as skeletal remains, sometimes also as traces from borings in shells or as immurations. The ancestors of the orders represented in the Early Ordovician are assumed to have been uncalcified. Five of the Ordovician orders belonged to the class Stenolaemata, characterised by mostly tubular and non-operculate zooids and brood-chambers for reproduction. Only one of those orders, the Cyclostomata, survived to the present day. The sixth order, evidenced by borings in molluscan shells, was the Ctenostomata. It survived to the present day and, in the latest Jurassic, generated a clade that burgeonned during the Late Cretaceous and Cenozoic to become the largest living bryozoan order, the Cheilostomata. Ctenostomes and cheilostomes together comprise the class Gymnolaemata, with mostly box-like zooids that are operculate in the cheilostomes. Today, the phylum Bryozoa comprises about 15000 fossil species and 6000 living species.