With Figure 10.

A seaweed-like Polyzoon, universal in European seas and widely spread over the world. It is colonial, like the vast majority of Polyzoa, and the colony or zoarium forms erect fronds which spring from a small basal portion, spread over some foreign object in the fashion of a Membrani-pora. The fronds vary in outline in different specimens: they are flexible owing to the small amount of calcareous matter deposited in the chitinous walls of the zooecium, i.e. the resistent part of the ectocyst of the individuals making up the colony. They are composed of two lamellae, in which the zooecia are placed back to back. Some species of the genus, however, have the zooecia disposed in only one lamella. The zooecia themselves are arranged in parallel longitudinal rows, and the zooecia in one row alternate in position with the zooecia of the adjoining rows to either side. An individual zooecium is elongate in shape, with one, the distal, end curved or rounded, and the other, the proximal, end somewhat contracted. The margin of the rounded distal end bears two spines to either side the middle line, and in some cases a fifth single spine placed centrally.

A whitish spot near this end, readily visible under a lens, marks the position of the retracted forepart of the individual or tentacle-sheath which bears the circlet of tentacles. New zooecia are added to the colony at the curved edges of the fronds.

Flustra foliacea generally grows upon stones or shells at moderate depths, but may occur in deeper water (sixty-two to seventy fathoms). It possesses a strong and peculiar smell when first taken from the water, and its fronds form a favourite resting place for other Polyzoa and various Hydroids.

The anatomy of Membranipora (Flustra) membranacea has been well studied by Nitsche, and is illustrated in Fig. 10 A. It is abundant and universal on our coasts. The colony is composed of a single lamella in which the zooecia are disposed in parallel series, alternating as in Flustra foliacea. The zoarium is attached to the fronds of Laminaria and Fucus, and may attain the length of several feet.

The zooecium or cell is in outline a parallelopiped. One surface, the lower, is attached; the other, the upper, is free. It has two elongated sides and two short ends. Of the two ends, one, that to the left in the figure, is proximal, i. e. near to the original parent zooid; the other distal, i. e. near to the growing free edge of the colony. A spine (Sp.) projects from the free surface at each of the two proximal angles. The mouth so-called, or aperture made by the retraction of the flexible fore-part or tentacle-sheath of the zooecium, is placed distally on the free surface. It is crescentic in outline, and its proximal edge or lip (op.) is thickened, forming the operculum, a structure from which the suborder Cheilostomata takes its name.

The wall of the zooecium is composed of a cuticle or ectocyst and an endo-cyst. The former (e.) is chitinoid, and the chitin of the base of the two spines, of the two ends with their angles, are strengthened by calcareous matter, and a calcareous plate also occupies the greater portion of each side. Each end is perforated in two places, each side in four places, by a set of pores. These pores are grouped within circular areae with raised margins, and are known as 'rosette' or 'communication' plates (r.). The endocyst (e.) is thin and delicate. Over the rosette plates it consists of columnar cells. Elsewhere it is membranous, containing scattered nuclei surrounded by masses of protoplasm. Two cords of fusiform cells, applied to the endocyst of the lower surface, connect the rosette plates at one end with the corresponding plates at the other end. These cords anastomose inter se, and with the lateral rosette plates. Other cords, the funiculi laterales (f.), composed of superficial fusiform cells surrounding a granular axis, run from one to the adjoining rosette plate, and are connected also to the funiculi of the stomach (f.). The fore-part of the body or tentacle-sheath1 when retracted is contained within the basal part. In Fig. 10, A. it is represented as fully expanded.

Its proximal portion is retained in a folded condition by certain ligaments and muscles. The summit of the fold forms a circular 'diaphragm' (d.), which contains a muscular sphincter, and closes over the tentacle-sheath (t.) when it is invaginated. The walls of the tentacle-sheath are homogeneous and contain nuclei, and are probably covered, as in other Polyzoa, by a delicate cuticle. It is crowned by a circlet of hollow, ciliated tentacles (T.), usually eighteen in number. The mouth (m.) lies in the centre of the circlet, and the cilia on the oral surfaces of the tentacles which work towards the mouth are longer than those on their aboral surfaces. The digestive tract consists of a more or less cup-shaped pharynx or oesophagus (o.) separated by a constriction from the stomach, which is divisible into a cardiac tubular portion (c.), a caecal sac (st.) and a pyloric region (P.), which is bent upwards on the distal or anal surface of the cardia. It ends with a short rectum (R.). The true anus (a.) is situated at the bottom of a tubular depression of the tentacle-sheath. The walls of the digestive tract are composed of a homogeneous lamella, lined internally by an epithelium, ciliated in the first part of the cardia, and the pylorus, while the non-ciliated cells of the stomach contain a brownish pigment; and coated externally by a layer of nucleated protoplasm, which passes at the base of the stomach into the fusiform cells of the funiculus (f'.) with its various branches.

These are connected to the funiculi laterales and the endo-cyst of the proximal wall of the zooecium. The nerve-ganglion (n.) is placed between the oesophagus and anus. According to Hincks (British Marine Polyzoa, i. Introduction, p. lxxxix), a ciliated intertentacular tube is often present in this species. Spermatozoa have been seen to pass outwards through it, but it is also present in the female zooid after Nitsche.

A. Longitudinal section, magnified, of Membranipora (Flustra) membranacea,

Fig. 10. A. Longitudinal section, magnified, of Membranipora (Flustra) membranacea

B. Avicularium (typical), after Hincks.

C. Vibraculum of Scrupocellaria scruposa, after Hincks.

1 The term 'polypide' is often used to denote 'the zooid, consisting of alimentary canal, with tentacles, nervous ganglion, etc, which is developed within the zooecium;' and then 'zooecium' ( = cell, auct; cystid, Nitsche; Brutkapsel, Reichert) denotes 'the chamber in which the polypide is lodged* (Hincks). The term zooecium is used here and in the general account of Polyzoa to denote the thickened part of the cuticle which persists after the death and decay of the rest of the organism. It is simply a skeletal structure. The term polypide is discarded altogether. For the view that a Polyzoon consists of two distinct animals, i.e. zooecium, and tentacle-sheath + digestive tract, is now known to be erroneous, and the use of the word only tends to maintain the error.