INTRODUCTION TO THE ECTOPROCTA

The entoprocts and ectoprocts collectively are known as the bryozoans. These are sessile (text with tooltip) Sessile is sedentary or attached to a substrate. organisms that usually live in dendritic colonies. Because many of them produced calcified structures and lived in large, reef-forming colonies, they have left an impressive fossil record. The ectoprocts have cup-shaped zoids which have ciliated tentacles (lophophores) that function to capture suspended food particles. The ectoprocts are distributed unevenly through three major groups: Phylactolaemata, Stenolaemata, and Gymnolaemata.

Phylactolaemata is found in freshwater only. The colonies may be sessile or somewhat mobile, but they are never mineralized. Individual zoids are monomorphic, or structurally identical. The lophophores are large and shaped like a horseshoe. The mouth area has an epistome (text with tooltip) The epistome is a part of the head in different arthropods. It is the region behind the mouth in insects and between the antennae and the mouth in crustaceans. It also is the preoral lobe of phoronids. , which is a special flap that overhangs the mouth and has an extension of the coelomic cavity, attributes also of the Phoronida. The coleomic cavities of the zoids in a colony are confluent. The zoids produce overwintering structures called statoblasts (text with tooltip) A statoblast is an asexual (and over-wintering) reproductive structure produced by certain ectoproct bryozoans. , which function also in dispersal. The statoblasts have a nonwetting surface that allows them to be caught in the surface film of the water and carried great distances.

Stenolaemata is a very small group of marine ectoprocts. The colonies are sessile and have a wall that is highly mineralized with calcium carbonate. Zoids do not have epistomes, nor do they have an operculum, a protective covering for the zoid when it retracts into its calcareous tube. The lophophore is circular.

Gymnolaemata is the most diverse group of bryozoans. Most taxa are marine, but some are freshwater forms. Usually, the wall is mineralized with calcium carbonate, and the deformation of the wall creates the hydrostatic pressure to extrude the lophophore. Zoids can be protected by an operculum. This group is highly polymorphic. There are the typical feeding zoids, which are similar to those of Stenolaemata (circular lophophore and no epistome). Then, there are other zoids that are highly modified, primarily for defense (a general term for these is heterozoids). The most unusual of the heterozoids is the avicularia, which has been modified to have a large hook-shaped operculum that snaps shut like a beak. These are defensive zoids that are highly muscular and can kill a predaceous worm or mollusk.

The ectoprocts resemble the entoprocts, but the ring of lophophores in the ectoprocts encloses only the mouth (not the mouth and anus as in the entoprocts). Also, the ectoprocts are eucoelomic unlike the entoprocts. The two groups resemble each other, but their relationships are not very clear. Brusca and Brusca (2003) consider them to occupy separate phyla, Tudge (2000) and Margulis and Schwartz (1998) do not. Nielsen (2001) does not believe that they (both groups) are related to the brachiopods and phoronids (the other lophophorates). He does, however, suggest that the two groups might have a sister group relationship. Molecular phylogenetic analyses (e.g. Fuchs et al. 2009) have confirmed the relationships within the major groups of the Ectoprocta. Furthermore, Hausdorf et al. (2007) and Helmkampf et al. (2008a and 2008b) used phylogenomic analyses and confirmed that Ectoprocta and Entoprocta form a monophyletic clade. From the discovery of Cycliophora, Funch and Kristensen (1995) suggested that they were related to the Entoprocta, and that proposal was confirmed Giribet et al. (2004) and Hejnol et al. (2009), thus uniting all three taxa into a higher taxon that they called Polyzoa.