30. Plumatella emarginata, Allman. ([Plate III], fig. 2; [plate IV], figs. 1, 1 a.)
Plumatella emarginata, Allman, Ann. Nat. Hist. xiii, p. 330 (1844). Plumatella emarginata, Johnston, Brit. Zooph. (ed. 2), p. 404 (1847). Alcyonella benedeni, Allman, Mon. Fresh-Water Polyzoa, p. 89, pl. iv, figs. 5-11 (1857). Plumatella emarginata, id., ibid. p. 104, pl. vii, figs. 5-10. Plumatella lucifuga, Jullien, Bull. Soc. zool. France, x, figs. 89, 90, p. 114 (1885). Plumatella princeps var. emarginata, Kraepelin (partim), Deutsch. Süsswasserbryoz. p. 120, pl. iv, fig. 108, pl. v, fig. 123 (1887). Plumatella emarginata, Braem, Unter. ii. Bryoz. süssen Wassers, p. 9, pl. i, figs. 12, 14 (Bibl. Zool. ii) (1890). Plumatella emarginata, Annandale (partim), J. As. Soc. Bengal, (new series) iii, 1907, p. 89. Plumatella princeps, Loppens (partim), Ann. Biol. lacustre, iii, p. 162, fig. 7 (1908). Plumatella emarginata, Annandale, Rec. Ind. Mus. v, p. 47 (1910).
Zoarium. The zoarium often covers a considerable area on flat surfaces and is sometimes entirely recumbent. More usually, however, the younger part is vertical. In either case the branching is practically dichotomous, two young zoœcia arising almost simultaneously at the tip of a mother-zoœcium and diverging from one another at a small angle. When the zoarium becomes vertical, rigid branches of as much as an inch in length are sometimes produced in this way and, arising parallel to one another, are pressed together to form an almost solid mass (=Alcyonella benedeni, Allman). In such cases the basal zoœcium or at any rate the basal part of each upright branch is considerably elongated. In recumbent zoœcia the main branches often radiate outwards from a common centre.
Zoœcia. The zoœcia are of almost equal width throughout, slender, and moderately elongate when recumbent. Their ectocyst is stiff; they are emarginate at the tip and more or less distinctly furrowed on the dorsal surface, the keel in which the furrow runs not being prominent. The orifice is often on the dorsal surface even in upright branches. Each zoœcium is of a dark brown or almost black colour for the greater part of its length but has a conspicuous white tip which is extended down the dorsal surface in the form of a triangle, its limits being rather more extensive than and parallel to those of the emargination.
Statoblast. The majority of the free statoblasts are elongate and truncate or subtruncate at the extremities, the sides being as a rule straight and parallel. In every polyparium specimens will be found that are between twice and thrice as long as broad. The capsule is, however, relatively much broader than the swim-ring, often being nearly circular, and there is therefore at either end a considerable extent of free air-cells, while the extent of these cells at the sides of the capsule is small. The air-cells cover a considerable part of the dorsal surface of the capsule. Fixed statoblasts are usually found in old colonies, especially at the approach of the hot weather. They have an oval form and are surrounded by a membranous margin on which traces of reticulation can often be detected. As a rule statoblasts of both types are produced in considerable but not in excessive numbers.
Polypide. There are about 40 tentacles, the velum at the base of which extends upwards for a considerable distance without being festooned. The stomach is elongate and slender and narrowly rounded at the base.
The method of branching, the coloration of the zoœcia and the form of the free statoblast are all characteristic. Luxuriant or closely compressed zoaria of P. diffusa often bear a superficial resemblance to those of P. emarginata, but the resemblance disappears if they are carefully dissected out. Indian specimens of P. emarginata agree closely with European ones.
Geographical Distribution.—P. emarginata is a common species in Europe, N. America, and southern Asia and probably also occurs in Africa and Australia. I have examined specimens from Calcutta, Rangoon, and Mandalay in Indian territory, and also from Jalor in the Patani States (Malay Peninsula) and the Talé Noi, Lakon Sitamarat, Lower Siam. Gemmules found by Apstein (Zool. Jahrb. (Syst.) xxv, 1907, p. 201) in plankton from the Colombo lake may belong to this species or to any of the others included by Kraepelin in his P. princeps.
Biology.—In Ireland Allan found P. emarginata in streams and rivulets, but it also occurs in European lakes. In India I have only found it in ponds. It prefers to adhere to the surface of stones or bricks, but when these are not available is found on the stems of water-plants. In the latter position the form called Alcyonella benedeni by Allman is usually produced, owing to the fact that the upright branches are crowded together through lack of space, very much in the same way (although owing to a different cause) as those of P. fruticosa are crowded together in the coralloides phase, to which the benedeni phase of P. emarginata is in many respects analogous.
Although it is essentially a cold-weather species in Calcutta, P. emarginata is sometimes found in a living condition during the "rains." Zoaria examined at this season, however, contains few living polypides, the majority of the zoœcia having rotted away and left fixed statoblasts only to mark their former position.