_Zooecium_ Those parts of the polyzoon-individual which constitute a case or "house" for the polypide.
SYNOPSIS OF THE CLASSIFICATION OF THE POLYZOA.
SYNOPSIS OF THE SUBCLASSES, ORDERS, AND SUBORDERS.
Small coelomate animals, each individual of which consists of a polyp-like organism or polypide enclosed in a "house" or zooecium composed partly of living tissues. The mouth is surrounded by a circle of ciliated tentacles that can be retracted within the zooecium; the alimentary canal, which is suspended in the zooecium, is Y-shaped and consists of three parts, the oesophagus, the stomach, and the intestine.
The anus as well as the mouth is enclosed in the circle of tentacles and the zooecium is not very distinctly separated from the polypide. Some forms are solitary or form temporary colonies by budding.
Most Entoprocta are marine, but a freshwater genus (_Urnatella_) occurs in N. America, while the Indian genus _Loxosomatoides_ (fig. 34, p. 176) is only known from brackish water.
The anus is outside the circle of tentacles and the zooecium can always be distinguished from the polypide. All species form by budding permanent communities the individuals in which remain connected together by living tissue.
Ectoproctous polyzoa the polypides of which have no epistome; the zooecia are in nearly all cases distinctly separated from one another by transverse perforated plates.
Most of the Gymnolæmata are marine, but species belonging to two of the three suborders into which they are divided often stray into brackish water, while a few genera that belong to one of these two suborders are practically confined to fresh water. The three suborders are distinguished as follows:--
The zooecia are provided with a "lip" or lid hinged to the posterior margin of the orifice (see fig. 33, p. 175). This lid closes automatically outside the zooecium or in a special chamber on the external surface (the "peristome") when the polypide retracts and is pushed open by the tentacles as they expand. The majority of the zooecia in each zoarium are more or less distinctly flattened, but some of them are often modified to form "vibracula" and "avicularia."
The Cheilostomata are essentially a marine group, but some species are found in estuaries and even in pools and ditches of brackish water (fig. 33).