A class of predominantly marine, colonial animals belonging to the phylum Bryozoa. Gymnolaemate zooids (individual colony members) are characterized by their short, wide, vaselike or boxlike zooecia (skeletal outer structures) and circular lophophores (food-gathering organs).
Classification and history
The gymnolaemates include several thousand species—mostly marine, some brackish, and a very few freshwater—belonging to the orders Ctenostomata and Cheilostomata. Appearing early in the Early Ordovician, when they included the probable stem group of the Ectoprocta, gymnolaemates remained quite inconspicuous until the Cretaceous, when they rose to the position of dominance among bryozoans which they still maintain. See also: Bryozoa; Cheilostomata; Ctenostomata
Highly varied in size and shape, most gymnolaemate colonies are small and delicate, but a few are large, conspicuous growths. Characteristics of these colonies remain essentially uniform throughout their extent (that is, they are not divisible into distinct endozone and exozone regions). The individual zooids may be relatively isolated, loosely grouped side by side, or tightly packed together.
The boxlike gymnolaemate zooecia lack internal calcareous cross-partitions like diaphragms. The zooecial wall may be a thin chitinous membrane (rarely coated externally with gelatinous material), a moderately thick and firm chitinous wall, or a thin to thick calcareous wall (covered externally by a thin chitinous cuticle). Just inside the zooecial wall is an epidermis, and immediately interior to that is a peritoneum, both composed of flat, thin cells; however, the body wall lacks a muscle layer. The zooecial aperture may be as wide as, but is often narrower than, the zooecium below.
An epistome (flap covering the mouth) is not present. The visceral cavities of adjacent zooids are not directly interconnected. The lophophore bears few to moderate numbers of tentacles (8–34, where counted). Individual gymnolaemate zooids are usually hermaphroditic, sometimes male or sometimes female. Individual colonies are always hermaphroditic, usually because they bear hermaphroditic zooids but sometimes because they bear both male and female zooids.
Asexual budding, which produces a new zooid within a gymnolaemate colony, begins by separating off a small sac- or boxlike portion of the parent zooid's visceral or body cavity. Cells along the inside of the body wall (cystids) then develop into a new set of internal soft-part organs (polypides) suspended in the newly separated sac or box. Periodically, the polypides of gymnolaemate zooids degenerate into brown bodies; the new succeeding polypides are regenerated from the original zooids' body walls. Only the very few freshwater gymnolaemates produce resistant resting bodies, known as hibernacula.