Geographical Distribution.—P. allmani is apparently a rare species to which there are few references in literature. It was originally described from England and is stated by Jullien to occur in France. I have found specimens only in the lake Bhim Tal (alt. 4,500 feet) in the W. Himalayas.
Biology.—The original specimens were found by Hancock on stones. My own were growing on the leaves of water-plants, usually on the under side. When the zoœcia were forced to stretch across from one leaflet to another they assumed the sinuous form characteristic of Allman's P. elegans.
34. Plumatella tanganyikæ, Rousselet.
Plumatella tanganyikæ, Rousselet, Proc. Zool. Soc. London, 1907 (i), p. 252, pl. xiv, figs. 1-4. Plumatella bombayensis, Annandale, Rec. Ind. Mus. ii, p. 169, figs. 1, 2 (1908). Plumatella bombayensis, id., ibid. v, p. 51 (1910).
Zoarium. The whole colony is recumbent but branches freely and at short intervals in a horizontal plane, so that the zoœcia become crowded together and the branches sometimes overlap one another. The zoarium often covers a considerable area, but growth seems to be mainly in two directions. When growing on the stems of water-plants the branches are often parallel and closely pressed together but remain recumbent in this position. A stout membrane sometimes extends between branches and individual zoœcia.
Zoœcia. The walls of the zoœcia are thick, stiff, and more or less darkly but not opaquely pigmented; the external surface, although not very smooth, is always clean. The two most noteworthy characters of the zoœcia are (i) their truncated appearance when the polypide is retracted, and (ii) the conspicuous, although often irregular external annulation of their walls. The tip of each zoœcium, owing to the fact that the invaginated part of the ectocyst is soft and sharply separated from the stiffened wall of the tube, terminates abruptly and is not rounded off gradually as is the case in most species of the genus; sometimes it expands into a trumpet-like mouth. The annulation of the external surface is due to numerous thickened areas of the ectocyst which take the form of slender rings surrounding the zoœcium; they are most conspicuous on its distal half. On the dorsal surface of the base of each zoœcium there is a conspicuous furrowed keel, which, however, does not usually extend to the distal end; the latter is oval in cross-section. The zoœcia are short and broad; their base is always recumbent, and, when the zoarium is attached to a stone or shell, often seems to be actually embedded in the support; the distal part turns upwards and is free, so that the aperture is terminal; the zoœcia of the older parts of the zoarium exhibit the specific characters much more clearly than those at the growing points.
Polypide. The lophophore bears 20 to 30 tentacles, which are long and slender; the velum at their base extends up each tentacle in the form of a sharply pointed projection, but these projections do not extend for more than one-fifth of the length of the tentacles. Both the velum and the tentacular sheath bear numerous minute tubercles on the external surface. The base of the stomach is rounded, and the whole of the alimentary canal has a stout appearance.
Fig. 44.—Plumatella tanganyikæ from Igatpuri Lake.
A=outline of part of zoarium from a stone, × 16; B=outline of the tip of a single zoœcium, × 70; C=free statoblast, × 70.