Bryozoa are trimeric, coelomate, colonial lophophorates. The sessile colonies are made up of zooids that remain in contact with each other through pores. Zooids are polymorphic, there are feeding zooids (autozooids) and specialised zooids (heterozooids). Colonies may be stolonic or crust-like; the zooids are calcified or uncalcified. Reproduction is asexually as well as sexually. Fertilised eggs develop into embryos (being a free-swimming form) that are retained and brooded in the mother zooid or in a sort of brood chamber, but emerge later as free-swimming larvae, or the embryos are freely spawn and undergo larval development outside, yet often still attached to the colony.
There are three recent bryozoan orders, the Cyclostomata, the Ctenostomata, and the Cheilostomata; all orders display breeding or immediate spawning of the embryos. Larvae may be short-lived and lecithotrophic as in the brooding species, or long-lived planktotrophic as in the species that shed their eggs directly in the sea water. Larval sizes range from 0.1 to little more than 1 mm.
¥ Order Cyclostomata. The larvae are remarkably similar small spheres (sizes a few tenths of millimeters) with slightly flattened oral poles and a densely ciliated outer cuticle. These larvae have a short free-swimming existence before they settle on the substratum. Due to their small size, cyclostomate larvae are not included in the present keys.
¥ Order Ctenostomata. The brooding species produce short-lived lecitotrophic larvae. The larva has a characteristic encircling band of ciliated cells, forming the corona. The body shape may be flattened or elongated. Larval sizes range from about 0.2 to 1.1 mm. The species that shed their eggs directly in the sea water produce long-lived planktotrophic larvae. These are enclosed by triangular bivalved shells of small size (0.1 mm). Coronate larvae are not keyed out here — if possible at all. An example is the coronata larva of Flustrella hispida [Flustr. hispida-larva] .
¥ Order Cheilostomata. Free spawning species produce the characteristic triangular cyphonautes larva. These larvae are long-lived and planktotrophic. The larval body is enclosed in a membranous shell; the size can be up to little over 1 mm. Cyphonautes larvae are not keyed out — if possible at all. An example is the cyphonautes larva of Membranopora membranacea [M.membranacea-larva] .
Ecology
Bryozoa are sessile in marine and freshwater habitats.
[After Hayward, 1985 (Ctenostomata); Hayward and Ryland, 1985 (Cyclostomata); Hayward and Ryland, 1995]