Freshwater Sponges, Hydroids & Polyzoa Part 46

_Zoarium._ The whole colony is rec.u.mbent but branches freely and at short intervals in a horizontal plane, so that the zooecia 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 rec.u.mbent in this position. A stout membrane sometimes extends between branches and individual zooecia.

_Zooecia._ The walls of the zooecia 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 zooecia 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 zooecium, owing to the fact that the inv.a.g.i.n.ated 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 zooecium; they are most conspicuous on its distal half. On the dorsal surface of the base of each zooecium there is a conspicuous furrowed keel, which, however, does not usually extend to the distal end; the latter is oval in cross-section. The zooecia are short and broad; their base is always rec.u.mbent, and, when the zoarium is attached to a stone or sh.e.l.l, often seems to be actually embedded in the support; the distal part turns upwards and is free, so that the aperture is terminal; the zooecia of the older parts of the zoarium exhibit the specific characters much more clearly than those at the growing points.

_Polypide._ The lophoph.o.r.e 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 ca.n.a.l has a stout appearance.

[Ill.u.s.tration: Fig. 44.--_Plumatella tanganyikae_ from Igatpuri Lake.

A=outline of part of zoarium from a stone, 16; B=outline of the tip of a single zooecium, 70; C=free statoblast, 70.]

_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. tanganyikae_, 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. tanganyikae_ 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 zooecia 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. tanganyikae_ 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 sh.e.l.ls, 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, _Hanc.o.c.k_. (Plate IV, fig. 5.)

_Plumatella punctata_, Hanc.o.c.k, 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. Ess.e.x 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. Ess.e.x 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.

Susswa.s.serbryozoen, i, p. 126, pl. iv, figs. 115, 116; pl.

v, figs. 124, 125; pl. vii, figs. 153, 154 (1887).

_Plumatella vesicularis_, Braem, Unters. u. Bryozoen sussen Wa.s.sers, 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 rec.u.mbent 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.

_Zooecia._ The zooecia differ from those of all other species in having a greatly swollen, soft ectocyst which can be transversely wrinkled all over the zooecium 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 zooecia 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 zooecia 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.

_Polypide._ The polypide is comparatively short and stout. European specimens are said to have from 30 to 40 tentacles, but Indian specimens have only from 20 to 30.

Shrunken specimens of the less congested forms of this species closely resemble specimens of _P. repens_, but the statoblasts are more variable in shape and the ectocyst, even in such specimens, is thicker. Living or well-preserved specimens cannot be mistaken for those of any other species. Jullien regarded _P. punctata_ as the type of a distinct genus (_Hyalinella_) but included in _Plumatella_ at least one form (P.

"_arethusa_") which probably belongs to this species. Kraepelin distinguishes as "varieties" two phases, a summer phase ("var.

_prostrata_") and an autumn phase ("var. _densa_"). The former often forms linear series of considerable length with only an occasional side-branch, while in the autumn phase branching is so profuse and the branches are so closely pressed together that the zoarium comes to resemble a uniform gelatinous patch rather than a dendritic growth. A phase resembling the European autumn form is the commonest in Calcutta and I have also found one intermediate between this and Kraepelin's "var. _prostrata_," neither having any seasonal significance in India.

GEOGRAPHICAL DISTRIBUTION.--_P. punctata_ is widely distributed in Europe and N. America, but in the Oriental Region it has only been found in Calcutta and the neighbourhood.

BIOLOGY.--In this part of India _P. punctata_ flourishes both during the "rains" and in winter. I have found specimens in June and July and also in December and January. The majority of them were attached to bricks, but some were on the roots of duckweed, the stems of water-plants, and the tips of creepers falling into water. The species is often found together with _Stolella indica_ and also with other species of its own genus. It is most common, in the neighbourhood of Calcutta, in that part of the town which is near the Salt Lakes, and occurs in ponds the water of which is slightly brackish.

Genus 2. STOLELLA, _Annandale_.

_Stolella_, Annandale, Rec. Ind. Mus. iii, p. 279 (1909).

_Stolella_, _id._, _ibid._ v, p. 53 (1910).

TYPE, _Stolella indica_, Annandale.

_Zoarium_. The zoarium consists of groups of zooecia (or occasionally of single zooecia) joined together by an adherent rhizome. There is no gelatinous investment.

_Zooecia._ The adult zooecia resemble those of _Plumatella_ except in being sometimes more or less upright.

_Polypide_ and _Statoblasts._ The polypide and statoblasts resemble those of _Plumatella_. Fixed as well as free statoblasts occur.

This genus is closely allied to _Plumatella_, from which it is probably derived. The root-like tube from which the zooecia arise is formed by the great elongation of the basal part of a zooecium, and the zoaria closely resemble those of _P. punctata_, for it is not until several zooecia have been produced that the characteristic mode of growth becomes apparent.

_Stolella_ has only been found in India and is monotypic[BJ].

[Footnote BJ: But see p. 246 (addenda).]

36. Stolella indica, _Annandale_. (Plate V, figs. 3, 4.)

_Stolella indica_, Annandale, Rec. Ind. Mus. iii, p. 279, fig. (1909).

_Stolella indica_, _id._, _ibid._ v, p. 53 (1910).

_Zoarium._ The zoarium is adherent and linear, having neither lateral nor vertical branches.