Freshwater Sponges, Hydroids & Polyzoa Part 44

_Statoblasts._ Both free and stationary statoblasts are formed, but the latter are rare and do not always adhere. They resemble the free statoblasts in general form but have a solid margin instead of a swim-ring and are often minutely serrated round the edge. The free statoblasts are at least considerably, sometimes very elongate; in all zoaria it is possible to find specimens that are more than twice as long as broad. The capsule is relatively large and resembles the swim-ring in outline, so that the free portion of the latter is not much narrower at the sides than at the ends. The sides are distinctly convex and the ends rounded; the swim-ring encroaches little on the surface of the capsule.

_Polypide._ The tentacles number between 40 and 50 and are not festooned at the base. The stomach is slender and elongate.

TYPE not in existence.

SYSTEMATIC REMARKS.--_P. fruticosa_ is closely allied to _P. repens_ (European and N. American) but always has much longer statoblasts. Three phases of the species may be distinguished as follows:--

A. (_Forma typica_). Zooecia stout in form, not greatly elongate; free branches produced in profusion.

B. (_P. stricta_, Allman, _P. repens_, van Beneden). Zooecia slender; free branches absent or consisting of two or three zooecia only.

C. (_P. coralloides_, Allman). Vertical zooecia pressed together and greatly elongated.

Indian specimens of the typical form agree well with German specimens labelled by Prof. Kraepelin _P. princeps_ var. _fruticosa_, and specimens of the _coralloides_ phase could hardly be distinguished from similar specimens from Scotland.

GEOGRAPHICAL DISTRIBUTION.--_P. fruticosa_ is widely distributed in Europe and probably in N. America. I have seen Indian specimens from the Punjab (Lah.o.r.e, _Stephenson_), from Bombay, from Travancore, from Calcutta and other places in the Ganges delta, from Rajshahi (Rampur Bhoolia) on the R. Ganges, from Kurseong in the E. Himalayas (alt. 4,500 feet), and from k.a.w.kareik in Tena.s.serim. Statoblasts found on the surface of a pond near Simla in the W. Himalayas (alt. _ca._ 8,000 feet), probably belong to this species.

BIOLOGY.--Allman states that in England _P. fruticosa_ is fond of still and slowly-running water. The typical form and the _coralloides_ phase grow abundantly in the Calcutta tanks, the former often attaining an extraordinary luxuriance. I have found the var. _stricta_ only in water in which there was reason to suspect a lack of minute life (and therefore of food), viz. in Shasthancottah Lake in Travancore, in a swamp in Lower Burma, and in a small jungle stream near the base of the Western Ghats in Travancore. The species is the only one that I have seen in running water in India, and the specimens obtained in the jungle stream in Travancore are the only specimens I have taken in these circ.u.mstances. _P. fruticosa_ always grows near the surface or near the edge of water; it is found attached to the stems of bulrushes and other aquatic plants, to floating seeds and logs and (rarely) to stones and bricks. So far as my experience goes it is only found, at any rate in Calcutta, in the cold weather and does not make its appearance earlier than October.

The form Allman called _P. coralloides_ was found by him, "attached to floating logs of wood, together with _P. repens_ and _Cordylophora lacustris_, and generally immersed in ma.s.ses of _Spongilla fluviatilis_." I have always found it immersed in sponges (_S.

lacustris_, _S. alba_, _S. carteri_, and _S. cra.s.sissima_), except when the sponge in which it had been immersed had decayed. Indeed, the peculiar form it has a.s.sumed appears to be directly due to the pressure of the growing sponge exerted on the zooecia, for it is often possible to find a zoarium that has been partially overgrown by a sponge and has retained its typical form so long as it was free but has a.s.sumed the _coralloides_ form where immersed.[BI] In Shasthancottah Lake, Travancore, I found specimens of the _stricta_ phase embedded in the gelatinous ma.s.s formed by a social rotifer and to some extent a.s.similated to the _coralloides_ form.

[Footnote BI: Braem (_op. cit._, p. 3, pl. i, fig. 1), has described and figured under the name _P. fungosa_ var.

_coralloides_, Allman, a dense form that somewhat resembles this phase of _P. fruticosa_ but has become compacted without external pressure. It is, however, probably a form of _P. repens_ rather than _P. fungosa_ and differs in its broad statoblasts from any form of _P. fruticosa_. I have examined specimens of the same form from England.]

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. Susswa.s.serbryoz. p. 120, pl. iv, fig.

108, pl. v, fig. 123 (1887).

_Plumatella emarginata_, Braem, Unter. ii. Bryoz. sussen Wa.s.sers, 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 rec.u.mbent. More usually, however, the younger part is vertical. In either case the branching is practically dichotomous, two young zooecia arising almost simultaneously at the tip of a mother-zooecium 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 ma.s.s (=_Alcyonella benedeni_, Allman). In such cases the basal zooecium or at any rate the basal part of each upright branch is considerably elongated. In rec.u.mbent zooecia the main branches often radiate outwards from a common centre.

_Zooecia._ The zooecia are of almost equal width throughout, slender, and moderately elongate when rec.u.mbent. 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 zooecium 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 zooecia 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 Tale 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 s.p.a.ce, 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 a.n.a.logous.

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 zooecia having rotted away and left fixed statoblasts only to mark their former position.

31. Plumatella javanica, _Kraepelin_.

_Plumatella javanica_, Kraepelin, Mitt. Nat. Mus. Hamb.

xxiii, p. 143, figs. 1-3 (1903).

_Plumatella emarginata_ var. _javanica_, Loppens, Ann. Biol.

lacustre, iii, p. 162 (1908).

_Plumatella javanica_, Annandale, Rec. Ind. Mus. v, p. 50 (1910).

_Plumatella allmani_ var. _dumortieri_, _id._ (_partim_) (_nec_ Allman), _ibid._ p. 49.

This species is related to _P. emarginata_, from which it may be distinguished by the following characters:--

_Zoarium._ The zoarium is always entirely rec.u.mbent and branches sparingly; its method of branching does not approach the dichotomous type but is lateral and irregular. Linear series of zooecia without lateral branches are often formed.

_Zooecia._ The zooecia are slender and often very long; they are strongly emarginate and furrowed, and the keel that contains the furrow is conspicuous. The ectocyst is hyaline and as a rule absolutely colourless.

_Statoblasts._ The free statoblasts are variable in length, sometimes distinctly elongate, sometimes elongate only to a moderate degree; they are rounded at the extremities and have the sides slightly or distinctly convex outwards. The capsule is relatively large, and the free portion of the swim-ring is not much broader at the ends than at the sides. The fixed statoblasts are elongate and surrounded by an irregularly shaped chitinous membrane, which is often of considerable extent. The whole of the dorsal surface is covered with what appear to be rudimentary air-s.p.a.ces some of which even contain air.