Statoblasts. Both fixed and free statoblasts are produced, but not in very large numbers. The latter are broadly oval and are surrounded by a stout chitinous ring, which often possesses irregular membranous projections; the surface is smooth. The free statoblasts are small and moderately elongate, the maximum breadth as a rule measuring about 2/3 of the length; the capsule is relatively large and the ring of air-cells is not very much broader at the ends than at the sides; the dorsal surface of the central capsule is profusely tuberculate. The outline of the whole structure is often somewhat irregular.
In deference to Mr. Rousselet's opinion expressed in a letter I have hitherto regarded the Bombay form of this species as distinct from the African one, and there certainly is a great difference in the appearance of specimens taken on the lower surface of stones in Igatpuri Lake and of the types of P. tanganyikæ, one of which is now in the collection of the Indian Museum. The dark colour of the former, however, and their vigorous growth appear to be directly due to environment, for these characters disappear to a large extent in specimens growing on the stems of water-plants in the same lake. Indeed, such specimens are exactly intermediate between the form "bombayensis" and the typical form of the species. P. tanganyikæ is closely allied to P. philippinensis, Kraepelin, from the island of Luzon, but the latter has a smooth and polished ectocyst devoid of annulations, and zoœcia of a more elongate and regular form.
Types of the species in the British and Indian Museums, those of P. bombayensis in the latter collection.
Geographical Distribution.—P. tanganyikæ is only known as yet from L. Tanganyika in Central Africa and from Igatpuri in the Bombay Presidency.
Biology.—In both localities the zoaria were found in shallow water. In L. Tanganyika they were encrusting stones and shells, while at Igatpuri they were fixed for the most part to the lower surface of stones but were also found on the stems of water-plants. My specimens from the Bombay Presidency were taken, on two separate occasions, at the end of November. At that date the zoaria were already decaying and large blanks, marked out by fixed statoblasts, were often observed on the stones. Probably, therefore, the species flourishes during the "rains."
35. Plumatella punctata, Hancock. ([Plate IV], fig. 5.)
Plumatella punctata, Hancock, Ann. Nat. Hist. (2) v, p. 200, pl. iii, fig. 1, and pl. v, figs. 6, 7 (1850). Plumatella vesicularis, Leidy, P. Ac. Philad. vii, p. 192 (1854). Plumatella vitrea, Hyatt, Comm. Essex Inst. iv, pl. ix, figs. 1, 2 (1866). Plumatella punctata, Allman, Mon. Fresh-Water Polyzoa, p. 100, fig. 15 (1857). Plumatella vesicularis, id., ibid. p. 101. Plumatella vitrea, Hyatt, Proc. Essex Inst. v, p. 225, figs. 18, 19 (1868). Plumatella vesicularis, id., ibid. p. 225. Hyalinella vesicularis, Jullien, Bull. Soc. zool. France, x, p. 133, figs. 165-172 (1885). Hyalinella vitrea, id., ibid. p. 134, figs. 173-179. Plumatella punctata, Kraepelin, Deutsch. Süsswasserbryozoen, i, p. 126, pl. iv, figs. 115, 116; pl. v, figs. 124, 125; pl. vii, figs. 153, 154 (1887). Plumatella vesicularis, Braem, Unters. ü. Bryozoen süssen Wassers, p. 8, pl. i, fig. 8 (Bibl. Zool. ii) (1890). Hyalinella punctata, Loppens, Ann. Biol. lacustre, iii, p. 163 (1908). Plumatella punctata, Annandale, Rec. Ind. Mus. v, p. 52 (1910).
Zoarium. The zoarium is entirely recumbent and often appears to form an almost uniform flat layer instead of a dendritic body. Sometimes, however, it is distinctly linear, with lateral branches produced irregularly at considerable distances apart.
Zoœcia. The zoœcia differ from those of all other species in having a greatly swollen, soft ectocyst which can be transversely wrinkled all over the zoœcium by the action of the muscles of the polypide and is distinctly contractile. It is mainly owing to the swollen and almost gelatinous nature of the ectocyst that the dendritic character of the zoarium is frequently concealed, for the method of branching is essentially the same as that of P. diffusa, although the zoœcia are not so distinctly elbowed. The ectocyst is colourless or faintly tinted with brown; as a rule it is not quite hyaline and the external surface is minutely roughened or tuberculate. The zoœcia are not emarginate or furrowed.
Statoblasts. Stationary statoblasts are not found. The free statoblasts are variable and often asymmetrical in outline, but the free portion of the swim-ring is always of nearly equal diameter all round the periphery and the capsule relatively large. Some of the statoblasts are always broad in comparison with their length.