_Zoarium_ The whole body of zooecia which are in organic connection.

_Zooecium_ Those parts of the polyzoon-individual which const.i.tute a case or "house" for the polypide.

SYNOPSIS OF THE CLa.s.sIFICATION OF THE POLYZOA.

I.

SYNOPSIS OF THE SUBCLa.s.sES, ORDERS, AND SUBORDERS.

Cla.s.s POLYZOA.

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 ca.n.a.l, which is suspended in the zooecium, is Y-shaped and consists of three parts, the oesophagus, the stomach, and the intestine.

Subcla.s.s ENTOPROCTA.

The a.n.u.s 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.

Subcla.s.s ECTOPROCTA.

The a.n.u.s 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.

Order I. GYMNOLaeMATA.

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 Gymnolaemata 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:--

Suborder A. _CHEILOSTOMATA._

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).

Suborder B. _CTENOSTOMATA._

The zooecia are provided with a collar-like membrane which is pleated vertically and closes together above the polypide inside the zooecium when the former is retracted; it is thrust out of the zooecium and expands into a ring-shaped form just before the tentacles are extruded.

The zooecia are usually more or less tubular, but in some genera and species are flattened.

The majority of the Ctenostomata are marine, but some genera are found in estuaries, while those of one section of the suborder live almost exclusively in fresh water.

Suborder C. _CYCLOSTOMATA._

The zooecia are provided neither with a lip nor with a collar-like membrane. They are tubular and usually have circular orifices.

The Cyclostomata are exclusively marine.

Order II. PHYLACTOLaeMATA.

Ectoproctous polyzoa the polypides of which have a leaf-shaped organ called an epistome projecting upwards and forwards within the circle of tentacles and between the mouth and the a.n.u.s. The zooecia are not distinct from one another, but in dendritic forms the zoarium is divided irregularly by chitinous part.i.tions.

The Phylactolaemata are, without exception, freshwater species.

II.

SYNOPSIS OF THE LEADING CHARACTERS OF THE DIVISIONS OF THE SUBORDER CTENOSTOMATA.

Suborder B. _CTENOSTOMATA._

The suborder has been subdivided in various ways by different authors.

The system here adopted is essentially the same as that proposed in a recent paper by Waters (Journ. Linn. Soc. London, Zool. xxi, p. 231, 1910), but I have thought it necessary to add a fourth division to the three adopted by that author, namely, the Alcyonellea, Stolonifera, and Vesicularina. This new division includes all the freshwater genera and may be known as the Paludicellina. In none of these divisions are the tentacles webbed at the base.

The four divisions may be recognized from the following synopsis of their characteristic features:--

Division I. ALCYONELLEA.

The zooecia arise directly from one another in a fleshy or gelatinous ma.s.s. The polypide has no gizzard. The species are essentially marine, but a few are found in brackish water in estuaries.

Division II. STOLONIFERA.

The zooecia arise from expansions in a delicate creeping rhizome or root-like structure, the order in which they are connected together being more or less irregular. As a rule (perhaps always) there is no gizzard. The species are marine.

Division III. VESICULARINA.

The zooecia grow directly from a tubular stem which is usually free and vertical, their arrangement being alternate, spiral or irregular. There is a stout gizzard which bears internal chitinous projections and is tightly compressed when the polypide is retracted. The species are essentially marine, but a few are found in brackish water.