In temperate climates statoblasts are produced in great profusion at the approach of winter, but in India they occur, in most species, in greatest numbers at the approach of the hot weather.
[Ill.u.s.tration: Fig. 31.--Part of the zoarium of _Victorella bengalensis_ entirely transformed into resting buds, 25. (From an aquarium in Calcutta.)]
In the family Paludicellidae (ctenostomata) external buds which resemble the statoblasts in many respects are produced at the approach of unfavourable climatic conditions, but no such buds are known in the family Hislopiidae, the zoaria of which appear to be practically perennial. The buds consist of ma.s.ses of cells formed at the points at which ordinary buds would naturally be produced, but packed with food-material and protected like statoblasts by a thick h.o.r.n.y coat. It seems also that old zooecia and polypides are sometimes transformed into buds of the kind (fig. 31), and it is possible that there is some connection between the formation of brown bodies and their production.
Like the statoblasts of the phylactolaemata the resting buds of the Paludicellidae are produced in Europe at the approach of winter, and in India at that of the hot weather.
DEVELOPMENT.
(a) _From the Egg._
Some polyzoa are oviparous, while in others a larva is formed within the zooecium and does not escape until it has attained some complexity of structure. Both the ctenostomatous genera that are found in fresh water in India are oviparous, but whereas in _Victorella_ the egg is small and appears to be extruded soon after its fertilization, in _Hislopia_ it remains in the zooecium for a considerable time, increases to a relatively large size, and in some unknown manner acc.u.mulates a considerable amount of food-material before escaping. Unfortunately the development is unknown in both genera.
In the phylactolaemata the life-history is much better known, having been studied by several authors, notably by Allman, by Kraepelin, and by Braem (1908). The egg is contained in a thin membrane, and while still enclosed in the zooecium, forms by regular division a hollow sphere composed of similar cells. This sphere then a.s.sumes an ovoid form, becomes covered with cilia externally, and breaks its way through the egg-membrane into the cavity of the zooecium. Inside the embryo, by a process a.n.a.logous to budding, a polypide or a pair of polypides is formed. Meanwhile the embryo has become distinctly pear-shaped, the polypide or polypides being situated at its narrow end, in which a pore makes its appearance. The walls are hollow in the region occupied by the polypide, the cavity contained in them being bridged by slender threads of tissue. The larva thus composed makes its way out of the zooecium, according to Kraepelin through the orifice of a degenerate bud formed for its reception, and swims about for a short time by means of the cilia with which it is covered. Its broad end then affixes itself to some solid object, the polypide is everted through the pore at the narrow end and the whole of that part of the larva which formerly enclosed it is turned completely inside out. A zoarium with its included polypides is finally produced from the young polypide by the rapid development of buds.
(b) _From the Statoblast and Resting Buds._
There is little information available as regards the development of the young polyzoon in the resting buds of the freshwater ctenostomes. In _Paludicella_ and _Pottsiella_ the capsule of the bud splits longitudinally into two valves and the polypide emerges between them; but in _Victorella bengalensis_ one of the projections on the margin of the bud appears to be transformed directly into the tip of a new zooecium and the capsule is gradually absorbed.
Contradictory statements have been made as regards several important points in the development of the statoblast and it is probable that considerable differences exist in different species. The following facts appear to be of general application. The cellular contents of the capsule consist mainly of a ma.s.s of cells packed with food-material in a granular form, the whole enclosed in a delicate membrane formed of flat cells. When conditions become favourable for development a cavity appears near one end of the ma.s.s and the cells that form its walls a.s.sume a columnar form in vertical section. The cavity increases rapidly in size, and, as it does so, a young polypide is budded off from its walls. Another bud may then appear in a similar fashion, and the zooecium of the first bud a.s.sumes its characteristic features. The capsule then splits longitudinally into two disk-like valves and the young polypide, in some cases already possessing a daughter bud, emerges in its zooecium, adheres by its base to some external object and produces a new polyparium by budding. The two valves of the statoblast often remain attached to the zoarium that has emerged from between them until it attains considerable dimensions (see Plate IV, fig. 3 _a_).
What conditions favour development is a question that cannot yet be answered in a satisfactory manner. Statoblasts can lie dormant for months and even for years without losing their power of germinating, and it is known that in Europe they germinate more readily after being subjected to a low temperature. In tropical India this is, of course, an impossible condition, but perhaps an abnormally high temperature has the same effect. At any rate it is an established fact that whereas the gemmules of most species germinate in Europe in spring, in Bengal they germinate either at the beginning of the "rains" or at that of our mild Indian winter.
MOVEMENTS.
[Ill.u.s.tration: Fig. 32.--Zoarium of _Lophopodella carteri_ moving along the stem of a water plant, 4. (From Igatpuri Lake.)]
In the vast majority of the polyzoa, marine as well as freshwater, movement is practically confined to the polypide, the external walls of the zooecium being rigid, the zooecia being closely linked together and the whole zoarium permanently fixed to some extraneous object. In a few freshwater species belonging to the genera _Cristatella_, _Lophopus_, _Lophopodella_ and _Pectinatella_, the whole zoarium has the power of progression. This power is best developed in _Cristatella_, which glides along with considerable rapidity on a highly specialized "sole" provided with abundant mucus and representing all that remains of the ectocyst.
It is by no means clear how the zoaria of the other genera move from one place to another, for the base is not modified, so far as can be seen, for the purpose, and the motion is extremely slow. It is probable, however, that progression is effected by alternate expansions and contractions of the base, and in _Lophopodella_ (fig. 32), which moves rather less slowly than its allies, the anterior part of the base is raised at times from the surface along which it is moving. The whole zoarium can be released in this way and occasionally drops through the water, and is perhaps carried by currents from one place to another in so doing.
So far as the polypides are concerned, the most important movements are those which enable the lophoph.o.r.e and the adjacent parts to be extruded from and withdrawn into the zooecium. The latter movement is executed by means of the retractor muscles, which by contracting drag the extruded parts back towards the posterior end of the endocyst, but it is not by any means certain how the extrusion of the lophoph.o.r.e is brought about.
In most ctenostomes the action of the parietal muscles doubtless a.s.sists in squeezing it out when the retractor and parieto-v.a.g.i.n.al muscles relax, but Oka states that protrusion can be effected in the phylactolaemata even after the zooecium has been cut open. Possibly some hydrostatic action takes place, however, and allowance must always be made for the natural resilience of the inverted portion of the ectocyst.
Even when the polypide is retracted, muscular action does not cease, for frequent movements, in some cases apparently rhythmical, of the alimentary ca.n.a.l may be observed, and in _Hislopia_ contraction of the gizzard takes place at irregular intervals.
When the lophoph.o.r.e is expanded, the tentacles in favourable circ.u.mstances remain almost still, except for the movements of their cilia; but if a particle of matter too large for the mouth to swallow or otherwise unsuitable is brought by the currents of the cilia towards it, individual tentacles can be bent down to wave it away and similar movements are often observed without apparent cause.
In the cheilostomes certain individuals of each zoarium are often profoundly modified in shape and function and exhibit almost constant rhythmical or convulsive movements, some ("avicularia") being shaped like a bird"s beak and snapping together, others ("vibracula") being more or less thread-like and having a waving motion.
DISTRIBUTION OF THE FRESHWATER POLYZOA.
Fifteen genera of freshwater Polyzoa are now recognized, one entoproctous and fourteen ectoproctous; five of the latter are ctenostomatous and nine phylactolaematous. Of the fourteen ectoproctous genera seven are known to occur in India, viz., _Victorella_, _Hislopia_, _Fredericella_, _Plumatella_, _Stolella_, _Lophopodella_, and _Pectinatella_. Except _Stolella_, which is only known from northern India, these genera have an extremely wide geographical range; _Victorella_ occurs in Europe, India, Africa, and Australia; _Hislopia_ in India, Indo-China, China, and Siberia; _Fredericella_ in Europe, N.
America, Africa, India, and Australia; _Plumatella_ in all geographical regions; _Lophopodella_ in E. and S. Africa, India, and j.a.pan; _Pectinatella_ in Europe, N. America, j.a.pan, and India.
Two genera, _Paludicella_ and _Lophopus_, have been stated on insufficient grounds to occur in India. The former is known from Europe and N. America, and is said to have been found in Australia, while the latter is common in Europe and N. America and also occurs in Brazil.
Of the genera that have not been found in this country the most remarkable are _Urnatella_ and _Cristatella_. The former is the only representative in fresh water of the Entoprocta and has only been found in N. America. Each individual is borne upon a segmented stalk the segments of which are enclosed in strong h.o.r.n.y coverings and are believed to act as resting buds. _Cristatella_, which is common in Europe and N. America, is a phylactolaematous genus of highly specialized structure. It possesses a creeping "sole" or organ of progression at the base of the zoarium.
The other phylactolaematous genera that do not occur in India appear to be of limited distribution, for _Australella_ is only known from N. S.
Wales, and _Stephanella_ from j.a.pan. The ctenostomatous _Arachnoidea_ has only been reported from Lake Tanganyika, and _Pottsiella_ only from a single locality in N. America.
As regards the exotic distribution of the Indian species little need be said. The majority of the _Plumatellae_ are identical with European species, while the only species of _Fredericella_ that has been discovered is closely allied to the European one. The Indian species of _Lophopodella_ occurs also in E. Africa and j.a.pan, while that of _Pectinatella_ is apparently confined to India, Burma and Ceylon, but is closely allied to a j.a.panese form.
POLYZOA OF BRACKISH WATER.
With the exception of _Victorella_, which occurs more commonly in brackish than in fresh water and has been found in the sea, the genera that occur in fresh water are confined or practically confined to that medium; but certain marine ctenostomes and cheilostomes not uncommonly make their way, both in Europe and in India, into brackish water, and in the delta of the Ganges an entoproctous genus also does so. The ctenostomatous genera that are found occasionally in brackish water belong to two divisions of the suborder, the Vesicularina and the Alcyonellea. To the former division belongs _Bowerbankia_, a form of which (_B. caudata_ subsp. _bengalensis_, p. 187) is often found in the Ganges delta with _Victorella bengalensis_. No species of Alcyonellea has, however, as yet been found in Indian brackish waters. The two Indian cheilostomes of brackish water belong to a genus (_Membranipora_) also found in similar situations in Europe. One of them (_M.
lacroixii_[AZ]) is, indeed, identical with a European form that occurs in England both in the sea and in ditches of brackish water. I have found it in the Cochin backwaters, in ponds of brackish water at the south end of the Chilka Lake (Ganjam, Madras), on the sh.o.r.e at Puri in Orissa, and in the Mutlah River at Port Canning. The second species (_M.
bengalensis_, Stoliczka) is peculiar to the delta of the Ganges[BA] and has not as yet been found in the open sea. The two species are easily recognized from one another, for whereas the lip of _M. bengalensis_ (fig. 33) bears a pair of long forked spines, there are no such structures on that of _M. lacroixii_, the dorsal surface of which is remarkably transparent. _M. lacroixii_ forms a flat zoarium, the only part visible to the naked eye being often the beaded margin of the zooecia, which appears as a delicate reticulation on bricks, logs of wood, the stems of rushes and of hydroids, etc.; but the zoarium of _M.
bengalensis_ is as a rule distinctly foliaceous and has a peculiar silvery l.u.s.tre.
[Footnote AZ: There is some doubt as to the proper name of this species, which may not be the one originally described as _Membranipora lacroixii_ by Andouin. I follow Busk and Hincks in my identification (see Cat. Polyzoa Brit. Mus. ii, p. 60, and Hist. Brit. Polyzoa, p. 129). Levinsen calls it _M. hippopus_, sp. nov. (see Morphological and Systematic Studies on the Cheilostomatous Bryozoa, p. 144; Copenhagen, 1909).]
[Footnote BA: Miss Thornely (Rec. Ind. Mus. i, p. 186, 1907) records it from Mergui, but this is an error due to an almost illegible label. The specimens she examined were the types of the species from Port Canning. Since this was written I have obtained specimens from Bombay--_April_, 1911.]
[Ill.u.s.tration: Fig. 33.--Outline of four zooecia of _Membranipora bengalensis_, Stoliczka (from type specimen, after Thornely). In the left upper zooecium the lip is shown open.]
_Loxosomatoides_[BB] (fig. 34), the Indian entoproctous genus found in brackish water, has not as yet been obtained from the open sea, but has recently been introduced, apparently from a tidal creek, into isolated ponds of brackish water at Port Canning. It is easily recognized by the chitinous shield attached to the ventral (posterior) surface.
[Footnote BB: Annandale, Rec. Ind. Mus. ii, p. 14 (1908).]
[Ill.u.s.tration: Fig. 34.--_Loxosomatoides colonialis_, Annandale.
A and B, a single individual of form A, as seen (A) in lateral, and (B) in ventral view; C, outline of a similar individual with the tentacles retracted, as seen from in front (dorsal view); D, ventral view of an individual and bud of form B. All the figures are from the type specimens and are multiplied by about 70.]
II.
HISTORY OF THE STUDY OF THE FRESHWATER POLYZOA.
The naturalists of the eighteenth century were acquainted with more than one species of freshwater polyzoon, but they did not distinguish these species from the hydroids. Trembley discovered _Cristatella_, which he called "Polype a Panache," in 1741, and Linne described a species of _Plumatella_ under the name _Tubipora repens_ in 1758, while ten years later Pallas gave a much fuller description (under the name _Tubularia fungosa_) of the form now known as _Plumatella fungosa_ or _P. repens_ var. _fungosa_. Although Trembley, Baker, and other early writers on the fauna of fresh water published valuable biological notes, the first really important work of a comprehensive nature was that of Dumortier and van Beneden, published in 1848. All previous memoirs were, however, superseded by Allman"s Monograph of the Fresh-Water Polyzoa, which was issued in 1857, and this memoir remains in certain respects the most satisfactory that has yet been produced. In 1885 Jullien published a revision of the phylactolaemata and freshwater ctenostomes which is unfortunately vitiated by some curious lapses in observation, but it is to Jullien that the recognition of the proper position of _Hislopia_ is due. The next comprehensive monograph was that of Kraepelin, which appeared in two parts (1887 and 1892) in the Abhandlungen des Naturwiss.
Vereins of Hamburg. In its detailed information and carefully executed histological plates this work is superior to any that preceded it or has since appeared, but the system of cla.s.sification adopted is perhaps less liable to criticism than that followed by Braem in his "Untersuchungen,"
published in the Bibliotheca Zoologica in 1888.
During the second half of the nineteenth century and the first decade of the twentieth several authors wrote important works on the embryology and anatomy of the phylactolaemata, notably Kraepelin, Braem, and Oka; but as yet the ctenostomes of fresh water have received comparatively little attention from anything but a systematic point of view.
From all points of view both the phylactolaemata and the ctenostomes of Asia have been generally neglected, except in the case of the j.a.panese phylactolaemata, which have been studied by Oka. Although Carter made some important discoveries as regards the Indian forms, he did not devote to them the same attention as he did to the sponges. In the case of the only new genus he described he introduced a serious error into the study of the two groups by placing _Hislopia_ among the cheilostomes, instead of in its true position as the type genus of a highly specialized family of ctenostomes.
For fuller details as to the history of the study of the freshwater Polyzoa the student may refer to Allman"s and to Kraepelin"s monographs.
An excellent summary is given by Harmer in his chapter on the freshwater Polyzoa in vol. ii. of the Cambridge Natural History; and Loppens has recently (1908) published in the Annales de Biologie lacustre a concise survey of the systematic work that has recently been undertaken.
Unfortunately he perpetuates Carter"s error as regards the position of _Hislopia_.
BIBLIOGRAPHY OF THE FRESHWATER POLYZOA.
A very full bibliography of the freshwater Polyzoa will be found in pt.