Plumatella diffusa, Leidy, P. Ac. Philad. v, p. 261 (1852). Plumatella diffusa, Allman, Mon. Fresh-Water Polyzoa, p. 105 (1857). Plumatella diffusa, Hyatt, Comm. Essex Inst. iv, pl. viii, figs. 11, 12 (1866). Plumatella diffusa, id., ibid. v, p. 107, fig. 12 (1868). Plumatella repens, Jullien, Bull. Soc. zool. France, x, fig. 37 (lapsus for 73), p. 110 (1885). Plumatella diffusa, id., ibid. figs. 155, 157, pp. 130, 131. Plumatella allmani var. diffusa, Annandale, Rec. Ind. Mus. v, p. 49 (1910).
Zoarium. The zoarium often covers a considerable area on flat surfaces and is sometimes found crowded together on the stems of plants. In the latter case the arrangement of the main branches is distinctly radiate. Upright branches occur rarely and never consist of more than three zoœcia. The characteristic method of branching is best represented by the following diagram:—
Fig. 43.
The partitions are stout and numerous.
Zoœcia. The great majority of the zoœcia in each zoarium are distinctly L-shaped, the long limb being usually adherent. The vital organs of the polypide are contained in the vertical limb, while the horizontal one, in mature polyparia, is packed full of free statoblasts. The zoœcia are cylindrical and as a rule obscurely emarginate and furrowed. The ectocyst is stiff; it is never deeply pigmented but is usually of a transparent horn-colour at the base of each zoœcium and colourless at the tip, the contrast between the two portions never being very strong. The basal portion is rough on the surface, the distal portion smooth.
Statoblasts. Free statoblasts are produced in very great profusion and fixed statoblasts are also to be found as a rule. The latter resemble those of P. emarginata. The free statoblasts are never very large or relatively broad, but they vary considerably as regards size and outline. The capsule is large, the sides convex outwards and the extremity more or less broadly rounded. The air-cells are unusually large and extend over a great part of the dorsal surface of the statoblast.
Polypide. The polypide is shorter and stouter than that of P. emarginata and as a rule has fewer tentacles.
The most characteristic feature of this species is the form of the zoœcia, which differ greatly from those of any other Indian species but P. allmani. In the latter they are distinctly "keg-shaped" (i. e., constricted at the base and swollen in the middle), and the zoarium never spreads out over large surfaces in the way in which that of P. diffusa does.
Type—? in the Philadelphia Academy of Sciences.