Certain forms of this genus are liable to become compacted together in such a way as to const.i.tute solid ma.s.ses consisting of elongate vertical zooecia closely parallel to one another and sometimes agglutinated by means of a gummy substance. These forms were given by Lamarck in 1816 the name _Alcyonella_, and there has been much dispute as to whether they represent a distinct genus, distinct species, or merely varieties or phases of more typical forms. It appears to be the case that all species which produce vertical branches are liable to have these branches closely packed together and the individual zooecia of which they are composed more or less greatly elongated. It is in this way that the form known to Allman as _Alcyonella benedeni_ is produced from the typical _Plumatella emarginata_. Other forms go further and secrete a gummy substance that glues the upright zooecia together and forces them to elongate themselves without branching. In these conditions the zooecia become polygonal in cross-section. It is probable that such forms (_e. g._, _Plumatella fungosa_ (Pallas)) should rank as distinct species, for the gummy secretion is present in great profusion even in young zoaria in which the zooecia have not yet a.s.sumed a vertical position. No such form, however, has as yet been found in India, and in any case it is impossible to regard _Alcyonella_ as a distinct genus.
_Key to the Indian Species of_ Plumatella.
I. Ectocyst more or less stiff, capable of transverse wrinkling only near the tips of the zooecia, never contractile or greatly swollen; zooecia rounded[BF] at the tip when the polypide is retracted. Free statoblasts elongate; the free portion of their swim-ring distinctly narrower at the sides than at the ends.
A. Ectocyst by no means rigid, of a uniform pale colour; zooecia never emarginate or furrowed, straight, curved or sinuous, elongate, cylindrical _fruticosa_, p. 217.
B. Ectocyst rigid; zooecia (or at any rate some of the zooecia) emarginate and furrowed.
_b_. Ectocyst darkly pigmented over the greater part of each zooecium, white at the tip; branching of the zoarium practically dichotomous, profuse, as a rule both horizontal and vertical; zooecia straight or slightly curved or sinuous _emarginata_, p. 220.
_b"_. Ectocyst colourless and hyaline; branching of the zoarium spa.r.s.e, lateral, irregular, horizontal; zooecia nearly straight, strongly emarginate and furrowed _javanica_, p. 221.
_b"_. The majority of the zooecia distinctly L-shaped, one limb being as a rule adherent; ectocyst never densely pigmented.
beta. Zooecia cylindrical, their furrowed keel never prominent _diffusa_, p. 223.
beta". Zooecia (or at any rate some of the zooecia) constricted or tapering at the base, their emargination and furrow conspicuous _allmani_, p. 224.
II. Ectocyst stiff; zooecia truncated when the polypide is retracted. Surface of zooecia minutely roughened, distinctly annulate on the distal part _tanganyikae_, p. 225.
III. Ectocyst swollen and contractile, capable of transverse wrinkling all over the zooecium; zooecia never emarginate _punctata_, p. 227.
[Footnote BF: In specimens preserved in spirit they are apt to collapse and therefore to become somewhat concave.]
There has always been much difficulty in separating the species of _Plumatella_, and even now there is no general consensus of opinion as to the number that should be recognized. The difficulty, however, is much reduced if the following precautions are observed:--
(1) If the zoarium appears to be tangled, if the branches intertwine or overlap, or if the zooecia are closely pressed together, the whole ma.s.s should be carefully dissected out.
This is necessary not only because zoaria belonging to different species are sometimes found entangled together but also because it is often difficult to recognize the characteristic method of branching and shape of the zooecia unless it is done.
(2) As large a part as possible of each zoarium should be examined, preferably with a binocular microscope, and allowance should be made for irregularities and abnormalities of all kinds. What must be observed is the rule rather than the exceptions.
(3) When the statoblasts are being examined, care must be taken that they lie flat and that their surface is parallel to that of the nose-piece of the microscope. If they are viewed obliquely it is impossible to see their true outlines and proportions.
(4) In order to see the relative proportions of the capsule and the swim-ring it is necessary that the statoblast should be rendered transparent. This is often difficult owing to the presence of air in the air-cells, but strong nitric acid applied judiciously will render it possible (p. 240).
In supervising the preparation of the plates that ill.u.s.trate this genus I have impressed upon the artist the importance of representing what he saw rather than what he thought he ought to see, and the figures are very close copies of actual specimens. I have deliberately chosen for representation specimens of _Plumatella_ preserved by the simple methods which are often the only ones that it is possible for a traveller to adopt, for the great majority of naturalists will probably have no opportunity of examining living specimens or specimens preserved by special methods, and the main object, I take it, of this series is to enable naturalists first to distinguish the species described and then to learn something of their habitat and habits.
GEOGRAPHICAL DISTRIBUTION.--Of the seven species included in this key five have been found in Europe (namely _P. fruticosa_, _P. emarginata_, _P. diffusa_, _P. allmani_, and _P. punctata_), while of these five all but _P. allmani_ are known to occur in N. America also. _P. javanica_ is apparently peculiar to the Oriental Region, while _P. tanganyikae_ has only been taken in Central Africa and in the Bombay Presidency.
TYPES.--Very few of the type-specimens of the older species of _Plumatella_ are in existence. Allman"s are neither in Edinburgh nor in London, and Mr. E. Leonard Gill, who has been kind enough to go through the Hanc.o.c.k Collection at Newcastle-on-Tyne, tells me that he cannot trace Hanc.o.c.k"s. Those of the forms described by Kraepelin are in Hamburg and that of _P. tanganyikae_ in the British Museum, and there are schizotypes or paratypes of this species and of _P. javanica_ in Calcutta. The types of Leidy"s species were at one time in the collection of the Philadelphia Academy of Science.
BIOLOGY.--The zoaria of the species of _Plumatella_ are found firmly attached to stones, bricks, logs of wood, sticks, floating seeds, the stems and roots of water-plants, and occasionally to the sh.e.l.ls of molluscs such as _Vivipara_ and _Unio_. Some species shun the light, but all are apparently confined to shallow water.
Various small oligochaete worms (e. g., _Chaetogaster spongillae_,[BG]
_Nais obtusa_, _Nais elinguis_, _Slavina appendiculata_ and _Pristina longiseta_[BH]), take shelter amongst them; dipterous larvae of the genus _Chironomus_ often build their protective tubes at the base of the zoaria, and the surface of the zooecia commonly bears a more or less profuse growth of such protozoa as _Vorticella_ and _Epistylis_. I have seen a worm of the genus _Chaetogaster_ devouring the tentacles of a polypide that had been accidentally injured, but as a rule the movements of the lophoph.o.r.e are too quick to permit attacks of the kind, and I know of no active enemy of the genus. The growth of sponges at the base of the zoaria probably chokes some species, but one form (_F.
fruticosa_) is able to surmount this difficulty by elongating its zooecia (p. 219). A small worm (_Aulophorus tonkinensis_) which is common in ponds in Burma and the east of India as far west as Lucknow, often builds the tube in which it lives mainly of the free statoblasts of this genus. It apparently makes no selection in so doing but merely gathers the commonest and lightest objects it can find, for small seeds and minute fragments of wood as well as sponge gemmules and statoblasts of other genera are also collected by it. I know of no better way of obtaining a general idea as to what sponges and phylactolaemata are present in a pond than to examine the tubes of _Aulophorus tonkinensis_.
[Footnote BG: Annandale, J. As. Soc. Bengal (n. s.) ii, p.
188, pl. i (1906).]
[Footnote BH: See Michaelsen, Mem. Ind. Mus. i, pp. 131-135 (1908).]
I am indebted to Mr. F. H. Gravely, a.s.sistant Superintendent in the Indian Museum, for an interesting note regarding the food of _Plumatella_. His observations, which were made in Northamptonshire, were unfortunately interrupted at a critical moment, but I have reproduced them with his consent in order that other observers may investigate the phenomena he saw. Mr. Gravely noted that a small green flagellate which was abundant in water in which _Plumatella repens_ was growing luxuriantly, was swallowed by the polypides, and that if the polyparium was kept in a shallow dish of water, living flagellata of the same species congregated in a little pile under the a.n.u.s of each polypide. His preparations show very clearly that the flagellates were pa.s.sing through the alimentary ca.n.a.l without apparent change, but the method of preservation does not permit the retractile granules, which were present in large numbers in the cell-substance of the flagellates, to be displayed and it is possible that these granules had disappeared from those flagellates which are present in the recta of his specimens.
It is clear, therefore, either that certain flagellates must pa.s.s through the alimentary ca.n.a.l of _Plumatella_ unchanged, or that the polyzoon must have the power of absorbing the stored food material the flagellates contain without doing them any other injury.
The free statoblasts of _Plumatella_ are as a rule set free before the cells they contain become differentiated, and float on the surface of the water for some time before they germinate; but occasionally a small polypide is formed inside the capsule while it is still in its parent zooecium. I have, however, seen only one instance of this premature development, in a single statoblast contained in a small zoarium of _P.
fruticosa_ found in Lower Burma in March. The fixed statoblasts usually remain fixed to the support of the zoarium, even when their parent-zooecium decays, and germinate _in situ_.
The larva (fig. 40 C, p. 207) that originates from the egg of _Plumatella_ is a minute pear-shaped, bladder-like body covered externally with fine vibratile threads (cilia) and having a pore at the narrow end. At the period at which it is set free from the parent zooecium it already contains a fully formed polypide or pair of polypides with the tentacles directed towards the narrow end. After a brief period of active life, during which it moves through the water by means of its cilia, it settles down on its broad end, which becomes adhesive; the polypide or pair of polypides is everted through the pore at the narrow end, the whole of this end is turned inside out, and a fresh polyparium is rapidly formed by budding.
29. Plumatella fruticosa, _Allman_. (Plate III, fig. 1; plate IV, fig.
4; plate V, fig. 1.)
_Plumatella fruticosa_, Allman, Ann. Nat. Hist. xiii, p. 331 (1844).
_Plumatella repens_, van Beneden (? _nec_ Linne), Mem. Acad.
Roy. Belg. 1847, p. 21, pl. i, figs. 1-4.
_Plumatella fruticosa_, Johnston, Brit. Zooph. (ed. 2), p.
404 (1847).
_Plumatella coralloides_, Allman, Rep. Brit. a.s.soc. 1850, p.
335.
_Plumatella stricta_, _id._, Mon. Fresh-Water Polyzoa, p.
99, fig. 14 (1857).
_Plumatella fruticosa_, _id._, _ibid._ p. 102, pl. vi, figs.
3-5.
_Plumatella coralloides_, _id._, _ibid._ p. 103, pl. vii, figs. 1-4.
_Plumatella repens_ and _P. stricta_, Carter, Ann. Nat.
Hist. (3) iii, p. 341 (1859).
_Plumatella lucifuga_, Jullien (_partim_), Bull. Soc. zool.
France, x, p. 114 (1885).
_Plumatella princeps_ var. _fruticosa_, Kraepelin, Deutsch.
Susswa.s.serbryozoen, i, p. 120, pl. vii, fig. 148 (1887).
_Plumatella fruticosa_, Braem, Unter. ii. Bryozoen des sussen Wa.s.sers, p. 9, pl. i, fig. 15 (Bibl. Zool. ii) (1890).
_Plumatella repens_, Annandale, J. As. Soc. Bengal (new series) iii, 1907, p. 88.
_Plumatella emarginata_, Loppens (_partim_), Ann. Biol.
lacustre, iii, p. 161 (1908).
_Plumatella fruticosa_, Annandale, Rec. Ind. Mus. v, p. 45 (1910).
_Zoarium._ The zoarium in the typical form has a loose appearance due to the fact that the branches are far apart and the ectocyst by no means rigid. When young the zoarium is adherent, but in well-grown polyparia vertical branches, often an inch or more in length, are freely produced.
As a rule they have not the strength to stand upright if removed from the water. Branching is ordinarily lateral and as a rule occurs chiefly on one side of a main branch or trunk. In certain circ.u.mstances upright zooecia are pressed together and reach a great length without branching, and in this form (_P. coralloides_, Allman) daughter-zooecia are often produced at the tip of an elongated mother-zooecium in fan-like formation. A depauperated form (_P. stricta_, Allman), occurs in which the vertical branches are absent or very short. In all forms internal part.i.tions are numerous and stout.
_Zooecia._ The zooecia are cylindrical and bear a simple keel on their dorsal surface. They are never emarginate or furrowed. In the typical form their diameter is more than half a millimetre, and they are always of considerable length. The ectocyst is thin and never very rigid or deeply pigmented, the colour usually being an almost uniform pale pinkish brown and fading little towards the tip of the zooecium.