Physical characteristics

Gymnolaemates are morphologically varied. The simplest have no skeletons, no polymorphs, and no change in zooidal form during colony development. The most complex gymnolaemates have elaborately calcified skeletons and at least two kinds of polymorphic zooids and display considerable variation in zooidal form during colony development (as-togeny). Zooids are cylindrical or flattened. The zoecium, which covers the zooid, consists of an organic cuticle composed of protein and chitin or of cuticle overlying calcium carbonate. In many species, the zoecium is heavy and rigid. Some impregnation of the chitinous layer with calcium carbonate may be present, even when a calcareous layer is ab sent. An orifice enables the lophophore (food-catching organ) to protrude. The lophophore is circular and consists of a simple ridge bearing eight to 30 or more tentacles. Just within the orifice is a chamber, the atrium. An epistome and intrinsic musculature in the body wall are lacking. Protrusion of the lophophore depends on body wall deformation. Within the body wall is a large coelom surrounding the U-shaped digestive tract. The mouth at the center of the lophophore opens into the digestive tract. The anus opens through the dorsal side of the tentacular sheath outside the lophophore, hence the name Ectoprocta ("outside anus"). Interzooidal communication occurs through a funicular network of tissue-plugged pores in vertical walls.

Gymnolaemates are colonial, sessile animals. Individuals composing the colonies usually are approximately 0.02 in (0.5 mm) long.

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