The individuals of the polyzoan colonies are small polyp-like organisms which have a cup-like inclosure of horny or of calcareous substance, or of the two combined, as mentioned above. This inclosing sheath is the exterior or cuticle of the animal and is called the /zo[oe]cium/, the whole colony being called the /zoarium/. The animal substance within the zo[oe]cium is the /polypide/, and consists of the mouth, tentacles, and alimentary system. A disk at the anterior end of the body, bearing the circle of tentacles, is [pg193] the /lophophore/; it is a contractile body, being, with the tentacles, protruded from the zo[oe]cium or drawn within it at will. In some species the orifices of the zo[oe]cia are surrounded with spines; others have lids, called /opercula/, which shut down when the polypides are retracted. A singular modification of the zo[oe]cia is seen in the genus /Bugula/, where peculiar shapes like birds' heads, called /avicularia/, occur. In these singular appendages the upper beak is hooked, while the lower one, or mandible, is the operculum of the zo[oe]cium. These constantly open and shut by means of muscles, and although their function is not certainly known, it is thought that they capture prey. Other modifications are whip-like processes, or flagellate filaments, called /vibracula/, which constantly beat the water. In many of the zo[oe]cia of the colony there are dark-colored masses, called /brown bodies/. These are formed by the degeneration of the polypides, which at certain periods break down, their tentacles and alimentary system ceasing to act and becoming brown masses. The body-wall then puts out a bud internally and forms a new polypide, which absorbs, or passes out, the brown body. The term of life of a single polypide is not known, but in every colony many such transformations are constantly taking place.
The colonies originally start from larvæ produced in the /ovicells/ of the zoaria. The ovicells are sometimes very large and pear-shaped, occurring at intervals on the stems; or there may be one at the top of each zo[oe]cium. They look like pearls. A larva, when liberated, swims about for a time, then attaches itself to a fixed object, and increases, by budding, into some one of the various forms which the colonies take.