Fig. 255.—Illustrating the nature of a secondary orifice (Cheilostomata). A, Mucronella coccinea Abildg., Scilly Is., × 40. The ovicell (o) overhangs the primary orifice, which is concealed by the great development of the peristome, produced into the mucro (mu); t, the three teeth (denticles) within the secondary orifice; a, avicularium. B, Porella compressa Sowb., Norway, × 40; p.o. primary orifice, above which is a concave lamina, the beginning of the ovicell. In the lower zooecium the ovicell (o) is further grown. The primary orifice is still visible, but it is partially concealed by the growth of the peristome, which encloses a minute avicularium; m, mandible of avicularium. C, Older part of the same colony; pr, peristome; s.o, secondary orifice; o', adult ovicell; p, pores.
In order to make preparations with the tentacles expanded, hydrochlorate of cocaine, chloral hydrate or spirit should be added gradually to the water. When the animals are completely anaesthetised they may be killed by means of a 7-10 p.c. solution of sulphate of copper (best made in distilled water or in rain water). This method gives admirable results in the case of both fresh-water and marine Polyzoa. The use of formaline (see p. [229]) may be strongly recommended for the Vesicularina.
The only recent work dealing with all the marine British forms is Mr. Hincks' invaluable History of the British Marine Polyzoa.[[599]] As the use of this book, unaided by any artificial help, is by no means easy to the beginner, the following key has been compiled as an index to the genera. The Entoproct forms, Loxosoma and Pedicellina (see pp. [488-491]), are not included in the table.
Fig. 256.—Illustrating the terminology of the front surface of the zooecium (Cheilostomata). A, Membranipora (Electra) pilosa L., Cromer, × 47; ap, the membranous "aperture;" o, orifice. B, Membranipora flemingii Busk, Plymouth, × 60; ap, the aperture, enclosed in a calcareous "area" (a); av, avicularium; s, marginal spines. C, Micropora coriacea Esper, Plymouth, × 43; a, area (calcareous); o, operculum; ov, ovicell.
In order to facilitate the use of the table here given in conjunction with Mr. Hincks' work, the nomenclature there adopted has been followed throughout. References to other descriptions of the species may be obtained by consulting Miss Jelly's admirable Synonymic Catalogue of the Recent Marine Bryozoa.[[600]]
Terminology.—A few technical terms must of necessity be employed. The colony is adherent when its zooecia are attached to the object on which the colony is growing. The zooecium is the body-wall of a single individual; and, except in transparent species, is the only part which can be seen from the outside in the retracted condition of the polypide or tentacles with the alimentary canal. The outermost layer of the zooecium is known as the ectocyst; it may be simply membranous, or calcified, or may be rendered opaque by foreign bodies; its surface in calcareous forms is often marked by pores (Fig. 239, C, p), which are vacuities in the calcareous wall, closed externally by membrane. A special median pore (Fig. 241, A, m.p) may occur, and is in some cases at least a complete perforation through the body-wall.
The tentacles are protruded through the orifice, which in Cheilostomata is usually guarded by a movable chitinous lid, or operculum (Fig. 256, A, o). Should the ectocyst be thickened or raised into a ridge surrounding the orifice, a tubular passage results, known as the secondary orifice (Fig. 255), at the deeper end of which is the true orifice. The peristome (Fig. 255, C, pr) is the raised or thickened part which gives rise to the secondary orifice. Should the zooecium be outlined by a raised ridge, the part so enclosed is known as the area (Fig. 256, C, a), if calcareous. The aperture or opesia (Fig. 256, A, B, ap) is a membranous part of the front surface; and may consist of the whole or part of the area. The orifice or the aperture is commonly provided with spines (Fig. 256, B, s).
Fig. 257.—A, Cribrilina annulata Fabr., Norway, × 33; c, calcareous bars concealing the membranous aperture: B, Membraniporella nitida Johnst., Plymouth, × 45; a, calcareous bars growing up round the margin of the aperture; b, the same, further developed; c, the same, completely formed (as in A); av, avicularium; o, immature, and o', mature, ovicell; s, marginal spines.