312 FOSSIL INVERTEBRATES/Bryozoans protected by a hinged flap (operculum) in some species Withdrawal is accomplished by the fast contraction of a retractor muscle running between the base of the lophophore and a point on the body wall Different taxonomic groups employ different mechanisms for protruding the lophophore However, all use a hydrostatic system entailing muscles acting on body walls to drive coelomic fluid into the tentacle sheath, forcing it to evert and push the lophophore out through the aperture of the zooid Bryozoans have a primitive nervous system Nervous activity can be demonstrated by touching a lophophore, thereby inducing its immediate retraction into the zooid The simultaneous retraction of the lophophores of adjacent zooids shows that the nervous systems of zooids are interconnected Zooids can also be linked by a system called the funiculus This is thought to be a homologue of the circulatory systems found in many other animals, but it does not carry blood or oxygen or have a heart Instead, the funiculus, which is linked to the stomachs of the zooids, transports lipids and other metabolites both within and between zooids Movement of food resources across the colony is undoubtedly important in the provisioning of nonfeeding zooids, developing larvae, and newly budded zooids at the growing edges of the colony Pores in the skeletal walls allow both nervous and funicular connections between zooids In some species soft-tissue linkages also occur over the outer ends of the skeletal walls between the zooids Reproduction and Growth The life cycles of bryozoans are complex and varied Colony growth occurs mostly by zooidal budding, an asexual process involving mitotic cell division However, the formation of new colonies is usually a sexual process, with meiotic cell division occurring in the testes and ovaries, which are located alongside the funiculus Bryozoan colonies are hermaphroditic, with each zooid usually producing sperm and eggs sequentially, although some species have separate male and female zooids Released through tiny pores in the tips of the tentacles, sperm are carried away in the exhalent feeding currents and ambient flow, and a small proportion survive to fertilize an egg in another colony Fertilized eggs develop into larvae In a minority of bryozoan species the larvae are planktotrophic – they feed while in the plankton for weeks or months However, most bryozoan species have non-planktotrophic larvae, which are brooded by the parent colony before being released into the plankton for a short period (hours or days) In both cases, the larvae eventually settle on a firm or hard surface (e.g a shell or stone) and undergo metamorphosis to form the first zooid (ancestrula) of a new colony Budding from the ancestrula produces the first generation of asexual zooids, which in turn bud further zooids, and so on New zooids are usually budded at specific locations, such as branch tips in tree-like colonies and the peripheral growing edge in sheet-like encrusting colonies (Figure 2B) The location and growth orientation of new buds largely determines the shape of the colony Modular zooids of similar shape can be ‘assembled’ into disparate colony forms, many of which have evolved in parallel in unrelated groups Various schemes have been devised to classify these growth forms, some using geometric terms (e.g reticulate) and others based on genera having the growth form (e.g reteporiform) A simple division is into encrusting, erect, and free-living colonies Encrusting colonies tend to be either sheet-like (Figure 2B), with zooids arranged multiserially, or Figure Bryozoan colony forms (A) Encrusting runner (cyclostome Voigtopora, Cretaceous, England, scale bar mm), (B) encrusting sheet (cheilostome Porella, Plio Pleistocene, New Zealand, scale bar mm), and (C) dome shaped colony (trepostome ‘Dianulites’, Ordovician, Russia, scale bar 10 mm)