Illustration: Radial sections of fragments of the skeletons of SpongillæRadial sections of fragments of the skeletons ofSpongillæ
Radial sections of fragments of the skeletons ofSpongillæ
A,S. crassissimavar.crassior(from Rajshahi); B,S. carteri(from Calcutta);a=transverse,b=radiating fibres;e=external surface of the sponge.
In the Spongillidæ the spicules and the skeleton are more important as regards the recognition of genera and species thanthe soft parts. The skeleton is usually reticulate, but sometimes consists of a mass of spicules almost without arrangement. The amount of spongin present is also different in different species. The spicules in a reticulate skeleton are arranged so as to form fibres of two kinds—radiating fibres, which radiate outwards from the centre of the sponge and frequently penetrate the external membrane, and transverse fibres, which run across from one radiating fibre to another. The fibres are composed of relatively large spicules (megascleres) arranged parallel to one another, overlapping at the ends, and bound together by means of a more or less profuse secretion of spongin. In some species they are actually enclosed in a sheath of this substance. The radiating fibres are usually more distinct and stouter than the transverse ones, which are often represented by single spicules but are sometimes splayed out at the ends so as to assume in outline the form of an hour-glass (fig. 3 B). The radiating fibres frequently raise up the membrane at their free extremities just as a tent-pole does a tent.
Normal spicules of the skeleton are always rod-like or needle-like, and either blunt or pointed at both ends; they are either smooth, granular, or covered with small spines. Sometimes spicules of the same type form a more or less irregular transverse network at the base or on the surface of the sponge.
Illustration: Part of an oscular collar of Spongilla lacustris subsp. reticulata, showing arrangement of microscleres in the derma (magnified).Fig. 4.—Part of an oscular collar ofSpongilla lacustrissubsp.reticulata, showing arrangement of microscleres in the derma (magnified).
Fig. 4.—Part of an oscular collar ofSpongilla lacustrissubsp.reticulata, showing arrangement of microscleres in the derma (magnified).
From the systematist's point of view, the structure of the free spicules found scattered in the substance and membrane of the sponge, and especially of those that form the armature of the gemmules, is of more importance than that of the skeleton-spicules. Free spicules are absent in many species; when present they are usually needle-like and pointed at the tips. In a few species, however, they are of variable or irregular form, or consist of several or many shafts meeting in a common central nodule. In one genus (Corvospongilla) they resemble a double grappling-iron in form, having a circle of strongly recurved hooks at both ends. The free microscleres, or flesh-spicules as they are often called, are either smooth, granular, or spiny.
Gemmule-spicules, which form a characteristic feature of the Spongillidæ, are very seldom absent when the gemmules are mature. They are of the greatest importance in distinguishing the genera. In their simplest form they closely resemble the free microscleres, but in several genera they bear, either at or near one end or at or near both ends, transverse disks which are either smooth or indented round the edge. In one genus (Pectispongilla) they are provided at both ends not with disks but with vertically parallel rows of spines resembling combs in appearance.
The simpler spicules of the Spongillidæ are formed in single cells (see fig. 2 E), but those of more complicated shape are produced by several cells acting in concert. Each spicule, although it is formed mainly of hydrated silica (opal), contains a slender organic filament running along its main axis inside the silica. This filament, or rather the tube in which it is contained, is often quite conspicuous, and in some species (e. g.,Spongilla crassissima) its termination is marked at both ends of the megasclere by a minute conical protuberance in the silica.
Unless sponges are alchemists and can transmute one element into another, the material of which the spicules are made must ultimately come from the water in which the sponges live, or the rocks or other bodies to or near which they are attached. The amount of water that must pass through a large specimen of such a sponge asSpongilla carteriin order that it may obtain materials for its skeleton must be enormous, for silica is an insoluble substance. I have noticed, however, that this sponge is particularly abundant and grows with special luxuriance in ponds in which clothes are washed with soap, and my friend Mr. G. H. Tipper has suggested to me that possibly the alkali contained in the soap-suds may assist the sponge in dissolving out the silica contained in the mud at the bottom of the ponds. The question of how the mineral matter of the skeleton is obtained is, however, one about which we know nothing definite.
The spongin that binds the skeleton-spicules together takes the form of a colourless or yellowish transparent membrane, which is often practically invisible. When very abundant it sometimes extends across the nodes of the skeleton as a delicate veil. In some sponges it also forms a basal membrane in contact with the object to which the sponge is attached, and in some such cases the spongin of the radiating fibres is in direct continuity with that of the basal membrane.
Colour and Odour.
Most freshwater sponges have a bad odour, which is more marked in some species than in others. This odour is not peculiar to the Spongillidæ, for it is practically identical with that given out by the common marine spongeHalichondria panicea.Its function is probably protective, but how it is produced we do not know.
The coloration of freshwater sponges is usually dull and uniform, butPectispongilla aureais of the brilliant yellow indicated by its name, while many species are of the bright green shade characteristic of chlorophyll, the colouring matter of the leaves of plants. Many species are brown or grey, and some are almost white.
These colours are due to one of three causes, or to a combination of more than one of them, viz.:—(1) the inhalation of solid inorganic particles, which are engulfed by the cells; (2) the presence in the cells of coloured substances, solid or liquid, produced by the vital activities of the sponge; and (3) the presence in the cells of peculiar organized living bodies known as "green corpuscles."
Sponges living in muddy water are often nearly black. This is because the cells of their parenchyma are gorged with very minute solid particles of silt. If a sponge of the kind is kept in clean water for a few days, it often becomes almost white. An interesting experiment is easily performed to illustrate the absorption and final elimination of solid colouring matter by placing a living sponge (small specimens ofSpongilla carteriare suitable) in a glass of clean water, and sprinkling finely powdered carmine in the water. In a few hours the sponge will be of a bright pink colour, but if only a little carmine is used at first and no more added, it will regain its normal greyish hue in a few days.
The colouring matter produced by the sponge itself is of two kinds—pigment, which is probably a waste product, and the substances produced directly by the ingestion of food or in the process of its digestion. When pigment is produced it takes the form of minute granules lying in the cells of the parenchyma, the dermal membrane being as a rule colourless. Very little is known about the pigments of freshwater sponges, and even less about the direct products of metabolism. It is apparently the latter, however, that give many otherwise colourless sponges a slight pinkish or yellowish tinge directly due to the presence in cells of the parenchyma of minute liquid globules. In one form ofSpongilla carterithese globules turn of a dark brown colour if treated with alcohol. The brilliant colour ofPectispongilla aureais due not to solid granules but to a liquid or semi-liquid substance contained in the cells.
The green corpuscles of the Spongillidæ are not present in all species. There is every reason to think that they represent a stage in the life-history of an alga, and that they enter the sponge in an active condition (see p. 49).
A fourth cause for the coloration of freshwater sponges may be noted briefly. It is not a normal one, but occurs commonly in certain forms (e. g.,Spongilla albavar.bengalensis). This cause is the growth in the canals and substance of the sponge of parasiticalgaæ, which turn the whole organism of a dull green colour. They do not do so, however, until they have reduced it to a dying state. The commonest parasite of the kind is a filamentous species particularly common in brackish water in the Ganges delta.
External Form and Consistency.
Illustration: Fig. 5.—Part of a type-specimen of Spongilla lacustris subsp. reticulata (nat. size).Fig. 5.—Part of a type-specimen ofSpongilla lacustrissubsp.reticulata(nat. size).
Fig. 5.—Part of a type-specimen ofSpongilla lacustrissubsp.reticulata(nat. size).
The external form of sponges is very variable, but each species, subspecies, or variety of the Spongillidæ has normally a characteristic appearance. The European race ofSpongilla lacustris, for example, consists in favourable circumstances of a flattened basal part from which long cylindrical branches grow out; while in the Indian race of the species these branches are flattened instead of being cylindrical, and anastomose freely. The structure of the branches is identical with that of the basal part. Many other species (for instance,Spongilla bombayensisandS. ultima) never produce branches but always consist of lichenoidor cushion-shaped masses. The appearance ofSpongilla crateriformis, when it is growing on a flattened surface which allows it to develop its natural form, is very characteristic, for it consists of little flattened masses that seem to be running out towards one another, just as though the sponge had been dropped, spoonful by spoonful, in a viscous condition from a teaspoon. Some species, such asTrochospongilla phillottiana, cover large areas with a thin film of uniform thickness, while others (e. g.,Spongilla albaandEphydatia meyeni) consist of irregular masses, the surface of which bears numerous irregular ridges or conical, subquadrate, or digitate processes. In a few forms (e. g.,Corvospongilla burmanica) the surface is covered with small turret-like projections of considerable regularity, and some (e. g.,Spongilla crassissima) naturally assume a spherical or oval shape with an absolutely smooth surface.
The production of long branches is apparently rare in tropical freshwater sponges.
The form of the oscula is characteristic in many cases. No other Indian species has them so large, or with such well-defined margins asSpongilla carteri(Pl. II, fig. 1). In many species (Pl. II, fig. 3) they have a stellate appearance owing to the fact that grooves in the substance of the sponge radiate round them beneath the external membrane. In other species they are quite inconspicuous and very small.
Illustration: Fig. 6.—Radial section through part of a dried sponge of Spongilla crassissima (from Calcutta), × 5.Fig. 6.—Radial section through part of a dried sponge ofSpongilla crassissima(from Calcutta), × 5.
Fig. 6.—Radial section through part of a dried sponge ofSpongilla crassissima(from Calcutta), × 5.
Spongillidæ differ greatly in consistency.Spongilla crassissimaandCorvospongilla lapidosaare almost stony, although the former is extremely light, more like pumice than true stone. Other species (e. g.,Trochospongilla latouchiana) are hard but brittle, while others again are soft and easily compressed, asSpongilla lacustris, the varietymollisofS. carteri, andS. crateriformis. The consistency of a sponge depends on two factors—the number of spicules present, and the amount of spongin. InCorvospongilla lapidosathe number of spicules is very large indeed. They are not arranged so as to form a reticulate skeleton but interlock inall directions, and there is hardly any spongin associated with them. InSpongilla crassissima, on the other hand, the number of spicules although large is not unusually so; but they form a very definitely reticulate skeleton, and are bound together by an unusually profuse secretion of spongin. InS. carterivar.mollisboth spicules and spongin are reduced to a minimum, and the parenchyma is relatively more bulky than usual.
Variation.
Sponges are very variable organisms, and even a slight change in the environment of the freshwater species often produces a considerable change in form and structure. Some species vary in accordance with the season, and others without apparent cause. Not only have many given rise to subspecies and "varieties" that possess a certain stability, but most if not all are liable to smaller changes that apparently affect both the individual and the breed, at any rate for a period.
(a)Seasonal Variation.
Weltner has shown in a recent paper (Arch. Natg. Berlin, lxxiii (i), p. 276, 1907) that in Europe those individuals ofEphydatiawhich are found (exceptionally) in an active condition in winter differ considerably both as regards the number of their cells and their anatomy from those found in summer. In Calcutta the majority of the individuals ofSpongilla carterithat are found in summer have their external surface unusually smooth and rounded, and contain in their parenchyma numerous cells the protoplasm of which is gorged with liquid. These cells give the whole sponge a faint pinkish tinge during life; but if it is plunged in spirit, both the liquid in the cells and the spirit turn rapidly of a dark brown colour. Specimens ofSpongilla crateriformistaken in a certain tank in Calcutta during the cold weather had the majority of the skeleton-spicules blunt, while the extremities of the gemmule-spicules were distinctly differentiated. Specimens of the same species taken from the same tank in July had the skeleton-spicules pointed, while the extremities of the gemmule-spicules were much less clearly differentiated. I have been unable to confirm this by observations made on sponges from other tanks, but it would certainly suggest that at any rate the breed of sponges in the tank first investigated was liable to seasonal variation.
(b)Variation due directly to Environment.
The characteristic external form of freshwater sponges is liable in most cases to be altered as a direct result of changes in theenvironment. The following are two characteristic instances of this phenomenon.
Certain shrubs with slender stems grow in the water at the edge of Igatpuri Lake. The stems of these shrubs support many large examples ofSpongilla carteri, which are kept in almost constant motion owing to the action of the wind on those parts of the shrubs that are not under water. The surface of the sponges is so affected by the currents of water thus set up against it that it is covered with deep grooves and high irregular ridges like cockscombs. Less than a hundred yards from the lake there is a small pond in whichSpongilla carteriis also abundant. Here it grows on stones at the bottom and has the characteristic and almost smooth form of the species.
My second instance also refers in part to Igatpuri Lake.Corvospongilla lapidosais common in the lake on the lower surface of stones, and also occurs at Nasik, about thirty miles away, on the walls of a conduit of dirty water. In the latter situation it has the form of large sheets of a blackish colour, with the surface corrugated and the oscula inconspicuous, while in the clear waters of the lake it is of a pale yellowish colour, occurs in small lichenoid patches, and has its oscula rendered conspicuous, in spite of their minute size, by being raised on little conical eminences in such a way that they resemble the craters of volcanoes in miniature.
Both the European and the Indian races ofSpongilla lacustrisfail to develop branches if growing in unfavourable conditions. In specimens obtained from the River Spree near Berlin these structures are sometimes many inches in length; while in mature specimens taken under stones in Loch Baa in the Island of Mull the whole organism consisted of a minute cushion-shaped mass less than an inch in diameter, and was also deficient in spicules. Both these breeds belong to the same species, and probably differ as a direct result of differences in environment.
(c)Variation without apparent cause.
Plate Iin this volume illustrates an excellent example of variation in external form to which it is impossible to assign a cause with any degree of confidence. The three specimens figured were all taken in the same pond, and at the same season, but in different years. It is possible that the change in form, which was not peculiar to a few individuals but to all those in several adjacent ponds, was due to a difference in the salinity of the water brought about by a more or less abundant rainfall; but of this I have been able to obtain no evidence in succeeding years.
Many Spongillidæ vary without apparent cause as regards the shape, size, and proportions of their spicules. This is the case as regards most species ofEuspongillaandEphydatia, and is a fact to which careful consideration has to be given in separating the species.
Nutrition.
Very little is known about the natural food of freshwater sponges, except that it must be of an organic nature and must be either in a very finely divided or in a liquid condition. The cells of the sponge seem to have the power of selecting suitable food from the water that flows past them, and it is known that they will absorb milk. The fact that they engulf minute particles of silt does not prove that they lack the power of selection, for extraneous matter is taken up by them not only as food but in order that it may be eliminated. Silt would soon block up the canals and so put a stop to the vital activity of the sponge, if it were not got rid of, and presumably it is only taken into the cells in order that they may pass it on and finally disgorge it in such a way or in such a position that it may be carried out of the oscula. The siliceous part of it may be used in forming spicules.
It is generally believed that the green corpuscles play an important part in the nutrition of those sponges in which they occur, and there can be no doubt that these bodies have the power peculiar to all organisms that produce chlorophyll of obtaining nutritive substances direct from water and carbonic oxide through the action of sunlight. Possibly they hand on some of the nourishment thus obtained to the sponges in which they live, or benefit them by the free oxygen given out in the process, but many Spongillidæ do well without them, even when living in identical conditions with species in which they abound.
Reproduction.
Both eggs and buds are produced by freshwater sponges (the latter rarely except by one species), while their gemmules attain an elaboration of structure not observed in any other family of sponges.
Probably all Spongillidæ are potentially monœcious, that is to say, able to produce both eggs and spermatozoa. In one Indian species, however, in which budding is unusually common (viz.Spongilla proliferens), sexual reproduction takes place very seldom, if ever. It is not known whether the eggs of sponges are fertilized by spermatozoa from the individual that produces the egg or by those of other individuals, but not improbably both methods of fertilization occur.
The egg of a freshwater sponge does not differ materially from that of other animals. When mature it is a relatively large spherical cell containing abundant food-material and situated in some natural cavity of the sponge. In the earlier stages of its growth, however, it exhibits amœboid movements, and makes its way through the common jelly. As it approaches maturity it is surrounded by other cells which contain granules of food-material. The food-material is apparently transferred by themin a slightly altered form to the egg. The egg has no shell, but in some species (e. g.Ephydatia blembingia[N]) it is surrounded, after fertilization, by gland-cells belonging to the parent sponge, which secrete round it a membrane of spongin. Development goes on within the chamber thus formed until the larva is ready to assume a free life.
The spermatozoon is also like that of other animals, consisting of a rounded head and a lash-like tail, the movements of which enable it to move rapidly through the water. Spermatozoa are produced inSpongillafrom spherical cells not unlike the eggs in general appearance. The contents of these cells divide and subdivide in such a way that they finally consist of a mass of spermatozoa surrounded by a single covering cell, which they finally rupture, and so escape.
Illustration: Fig. 7.—Diagram of a vertical section through the gemmule of Spongilla proliferens.Fig. 7.—Diagram of a vertical section through the gemmule ofSpongilla proliferens.
Fig. 7.—Diagram of a vertical section through the gemmule ofSpongilla proliferens.
A=cellular contents; B=internal chitinous layer; C=external chitinous layer; D=pneumatic coat; E=gemmule-spicule; F=external membrane; G=foraminal tubule.
Gemmules are asexual reproductive bodies peculiar to the sponges, but not to the Spongillidæ. They resemble the statoblasts of the phylactolæmatous polyzoa in general structure as well as in function, which is mainly that of preserving the race from destruction by such agencies as drought, starvation, and temperatures that are either too high or too low for its activities. This function they are enabled to perform by the facts that they are provided with coverings not only very hard but also fitted to resist the unfavourable agencies to which the gemmules are likelyto be exposed, and that they contain abundant food-material of which use can be made as soon as favourable conditions occur again.
Internally the gemmule consists of a mass of cells containing food-material in what may be called a tabloid form, for it consists of minutely granular plate-like bodies. These cells are enclosed in a flask-like receptacle, the walls of which consist of two chitinous layers, a delicate inner membrane and an outer one of considerable stoutness. The mouth of the flask is closed by an extension of the inner membrane, and in some species is surrounded by a tubular extension of the external membrane known as the foraminal tubule. Externally the gemmule is usually covered by what is called a "pneumatic coat," also of "chitin" (spongin), but usually of great relative thickness and honeycombed by spaces which contain air, rendering the structure buoyant. The pneumatic coat also contains the microscleres characteristic of the species; it is often limited externally by a third chitinous membrane, on which more gemmule-spicules sometimes lie parallel to the surface.
The cells from which those of the gemmules are derived are akin in origin to those that give rise to eggs and spermatozoa. Some zoologists are therefore of the opinion that the development of the gemmule is an instance of parthenogenesis—that is to say of an organism arising from an egg that has not been fertilized. But some of the collar-cells, although most of them originate from the external ciliated cells of the larva, have a similar origin. The building-up of the gemmule affords an excellent instance of the active co-operation that exists between the cells of sponges, and of their mobility, for the food-material that has to be stored up is brought by cells from all parts of the sponge, and these cells retire after discharging their load into those of the young gemmule.
The formation of the gemmule ofEphydatia blembingia, a Malayan species not yet found in India, is described in detail by Dr. R. Evans (Q. J. Microsc. Sci. London, xliv, p. 81, 1901).
Gemmules are produced by the freshwater sponges of Europe, N. America and Japan at the approach of winter, but in the tropical parts of India they are formed more frequently at the approach of the hot weather (p. 4). After they are fully formed the sponge that has produced them dies, and as a rule disintegrates more or less completely. In some species, however, the greater part of the skeleton remains intact, if it is not disturbed, and retains some of the gemmules in its meshwork, where they finally germinate. Other gemmules are set free. Some of them float on the surface of the water; others sink to the bottom. In any case all of them undergo a period of quiescence before germinating. It has been found that they can be kept dry for two years without dying.
The function of the special spicules with which the gemmulesof the Spongillidæ are provided appears to be not only to protect them but more especially to weight them to the extent suitable to the habits of each species. Species that inhabit running water, for example, in some cases have heavier gemmule-spicules than those that live in stagnant water, and their gemmules are the less easily carried away by the currents of the river. The gemmules of sponges growing in lakes are sometimes deficient in spicules. This is the case as regards the form ofSpongilla lacustrisfound in Lake Baa, Isle of Mull, as regardsS. helveticafrom the Lake of Geneva,S. mooreifrom Lake Tanganyika, andS. cogginifrom Tali-Fu in Yunnan; also as regards the species ofSpongillaandEphydatiafound in Lake Baikal, many of the sponges of which are said never to produce gemmules.
Except in the genusCorvospongillaand the subgenusStratospongilla, in both of which the air-spaces of the gemmules are usually no more than cavities between different chitinous membranes, the pneumatic coat is either "granular" or "cellular." Neither of these terms, however, must be understood in a physiological sense, for what appear to be granules in a granular coat are actually minute bubbles of air contained in little cavities in a foam-like mass of chitin (or rather spongin), while the cells in a cellular one are only larger and more regular air-spaces with thin polygonal walls and flat horizontal partitions. The walls of these spaces are said in some cases to contain a considerable amount of silica.
The gemmules with their various coverings are usually spherical in shape, but in some species they are oval or depressed in outline. They lie as a rule free in the substance of the sponge, but in some species adhere at its base to the object to which it is attached. In some species they are joined together in groups, but in most they are quite free one from another.
Reproductive buds[O]are produced, so far as is known, by very few Spongillidæ, although they are common enough in some other groups of sponges. In the only freshwater species in which they have been found to form a habitual means of reproduction, namely inSpongilla proliferens, they have much the appearance of abortive branches, and it is possible that they have been overlooked for this reason in other species, for they were noticed by Laurent inSpongilla lacustrisas long ago as 1840 (CR. Sé. Acad. Sci. Paris, xi, p. 478). The buds noticed by Laurent, however, were only produced by very young sponges, and were of a different nature from those ofS. proliferens, perhaps representing a form of fission rather than true budding (see 'Voyage de la Bonite: Zoophytologie,' Spongiaires, pl. i (Paris, 1844)).
InSpongilla proliferens, a common Indian species, the buds ariseas thickenings of the strands of cells accompanying the radiating spicule-fibres of the skeleton, which project outwards from the surface of the sponge. The thickenings originate beneath the surface and contain, at the earliest stage at which I have as yet examined them, all the elements of the adult organism (i. e.flesh-spicules, ciliated chambers, efferent and afferent canals, parenchyma-cells of various sorts) except skeleton fibres, gemmules, and a dermal membrane. A section at this period closely resembles one of an adult sponge, except that the structure is more compact, the parenchyma being relatively bulky and the canals of small diameter.
Laurent observed reproduction by splitting in young individuals ofSpongilla, but I have not been able to obtain evidence myself that this method of reproduction occurs normally in Indian species. In injured specimens ofSpongilla carteri, however, I have observed a phenomenon that seems to be rather an abnormal form of budding, little rounded masses of cells making their way to the ends of the radiating skeleton fibres and becoming transformed into young sponges, which break loose and so start an independent existence. Possibly the buds observed by Laurent inS. lacustriswere of a similar nature.
Development.
(a)From the Egg.
After fertilization, the egg, lying in its cavity in the sponge, undergoes a complete segmentation; that is to say, becomes divided into a number of cells without any residuum remaining. The segmentation, however, is not equal, for it results in the formation of cells of two distinct types, one larger and less numerous than the other. As the process continues a pear-shaped body is produced, solid at the broader end, which consists of the larger cells, but hollow at the other. Further changes result in the whole of the external surface becoming ciliated or covered with fine protoplasmic lashes, each of which arises from a single small cell; considerable differentiation now takes place among the cells, and spicules begin to appear. At this stage or earlier (for there seem to be differences in different species and individuals as to the stage at which the young sponge escapes) the larva makes its way out of the parent sponge. After a brief period of free life, in which it swims rapidly through the water by means of its cilia, it fixes itself by the broad end to some solid object (from which it can never move again) and undergoes a final metamorphosis. During this process the ciliated cells of the external layer make their way, either by a folding-in of the whole layer or in groups of cells, into the interior, there change into collar-cells and arrange themselves in special cavities—the ciliated chambers of the adult. Finally an osculum, pores, &c., are formed, and the sponge is complete.
This, of course, is the merest outline of what occurs; other changes that take place during the metamorphosis are of great theoretical interest, but cannot be discussed here. The student may refer to Dr. R. Evans's account of the larval development ofSpongilla lacustrisin the Q. J. Microsc. Sci. London, xlii, p. 363 (1899).
(b)From the Gemmule.
The period for which the gemmule lies dormant probably depends to some extent upon environment and to some extent on the species to which it belongs. Carter found that if he cleaned gemmules with a handkerchief and placed them in water exposed to sunlight, they germinated in a few days; but in Calcutta gemmules ofSpongilla albavar.bengalensistreated in this way and placed in my aquarium at the beginning of the hot weather, did not germinate until well on in the "rains." Even then, after about five months, only a few of them did so. Zykoff found that in Europe gemmules kept for two years were still alive and able to germinate.
Germination consists in the cellular contents of the gemmule bursting the membrane or membranes in which they are enclosed, and making their way out of the gemmule in the form of a delicate whitish mass, which sometimes issues through the natural aperture in the outer chitinous coat and sometimes through an actual rent in this coat. In the latter case the development of the young sponge is more advanced than in the former.
The fullest account of development from the gemmule as yet published is by Zykoff, and refers toEphydatiain Europe (Biol. Centralbl. Berlin, xii, p. 713, 1892).
His investigations show that the bursting of the gemmule is not merely a mechanical effect of moisture or any such agency but is due to development of the cellular contents, which at the time they escape have at least undergone differentiation into two layers. Of the more important soft structures in the sponge the osculum is the first to appear, the ciliated chambers being formed later. This is the opposite of what occurs in the case of the bud, but in both cases the aperture appears to be produced by the pressure of water in the organism. The manner and order in which the different kinds of cells originate in the sponge derived from a gemmule give support to the view that the primitive cell-layers on which morphologists lay great stress are not of any great importance so far as sponges are concerned.
(c)Development of the Bud.
As the bud ofSpongilla proliferensgrows it makes its way up the skeleton-fibre to which it was originally attached, pushing the dermal membrane, which expands with its growth, before it. Theskeleton-fibre does not, however, continue to grow in the bud, in which a number of finer fibres make their appearance, radiating from a point approximately at the centre of the mass. As the bud projects more and more from the surface of the sponge the dermal membrane contracts at its base, so as finally to separate it from its parent. Further details are given on p. 74.
Habitat.
Mr. Edward Potts[P], writing on the freshwater sponges of North America, says:—"These organisms have occasionally been discovered growing in water unfit for domestic uses; but as a rule they prefer pure water, and in my experience the finest specimens have always been found where they are subjected to the most rapid currents." True as this is of the Spongillidæ of temperate climates, it is hardly applicable to those of tropical India, for in this country we find many species growing most luxuriantly and commonly in water that would certainly be considered unfit for domestic purposes in a country in which sanitation was treated as a science. Some species, indeed, are only found in ponds of water polluted by human agency, and such ponds, provided that other conditions are favourable, are perhaps the best collecting grounds. Other favourable conditions consist in a due mixture of light and shade, a lack of disturbance such as that caused by cleaning out the pond, and above all in the presence of objects suitable for the support of sponges.
I do not know exactly why light and shade must be mixed in a habitat favourable for the growth of sponges, for most species prefer shade, if it be not too dense; but it is certainly the case that, with a few exceptions, Indian Spongillidæ flourish best in water shaded at the edges by trees and exposed to sunlight elsewhere. One of the exceptions to this rule is the Indian race ofSpongilla lucustris, which is found in small pools of water in sand-dunes without a particle of shade. Several species are only found on the lower surface of stones and roots in circumstances which do not suggest that their position merely protects them from mud, which, as Mr. Potts points out, is their "great enemy." A notable instance isTrochospongilla pennsylvanica, which is found hiding away from light in America and Europe as well as in India.
It is curious that it should be easy to exterminate the sponges in a pond by cleaning it out, for one would have thought that sufficient gemmules would have remained at the edge, or would have been brought rapidly from elsewhere, to restock the water. Mr. Green has, however, noted thatSpongilla carterihas disappeared for some years from a small lake at Peradeniya in which it was formerly abundant, owing to the lake having been cleanedout, and I have made similar observations on several occasions in Calcutta.
The question of the objects to which sponges attach themselves is one intimately connected with that of the injury done them by mud. The delta of the Ganges is one of the muddiest districts on earth. There are no stones or rocks in the rivers and ponds, but mud everywhere. If a sponge settles in the mud its canals are rapidly choked, its vital processes cease, and it dies. In this part of India, therefore, most sponges are found fixed either to floating objects such as logs of wood, to vertical objects such as the stems of bulrushes and other aquatic plants, or to the tips of branches that overhang the water and become submerged during the "rains." In Calcutta man has unwittingly come to the assistance of the sponges, not only by digging tanks but also by building "bathing-ghats" of brick at the edge, and constructing, with æsthetic intentions if not results, masses of artificial concrete rocks in or surrounding the water. There are at least two sponges (the typical form ofSpongilla albaandEphydatia meyeni) which in Calcutta are only found attached to such objects. The form ofS. alba, however, that is found in ponds of brackish water in the Gangetic delta has not derived this artificial assistance from man, except in the few places where brick bridges have been built, and attaches itself to the stem and roots of a kind of grass that grows at the edge of brackish water. This sponge seems to have become immune even to mud, the particles of which are swallowed by its cells and finally got rid of without blocking up the canals.
Several Indian sponges are only found adhering to stones and rocks. Among these speciesCorvospongilla lapidosaand our representatives of the subgenusStratospongillaare noteworthy. Some forms (e. g.Spongilla carteriandS. crateriformis) seem, however, to be just as much at home in muddy as in rocky localities, although they avoid the mud itself.
There is much indirect evidence that the larvæ of freshwater sponges exercise a power of selection as regards the objects to which they affix themselves on settling down for life.
Few Spongillidæ are found in salt or brackish water, butSpongilla albavar.bengalensishas been found in both, and is abundant in the latter; indeed, it has not been found in pure fresh water.Spongilla travancoricahas only been found in slightly brackish water, whileS. lacustrissubsp.reticulataandDosilia plumosaoccur in both fresh and brackish water, although rarely in the latter. The Spongillidæ are essentially a freshwater family, and those forms that are found in any but pure fresh water must be regarded as aberrant or unusually tolerant in their habits, not as primitive marine forms that still linger halfway to the sea.
Animals and Plants commonly associated with Freshwater Sponges.
(a)Enemies.
Freshwater sponges have few living enemies. Indeed, it is difficult to say exactly what is an enemy of a creature so loosely organized as a sponge. There can be little doubt, in any case, that the neuropteroid larva (Sisyra indica) which sucks the cells of several species should be classed in this category, and it is noteworthy that several species of the same genus also occur in Europe and N. America which also attack sponges. Other animals that may be enemies are a midge larva (Tanypussp.) and certain worms that bore through the parenchyma (p. 93), but I know of no animal that devours sponges bodily, so long as they are uninjured. If their external membrane is destroyed, they are immediately attacked by various little fish and also by snails of the generaLimnæaandPlanorbis, and prawns of the genusPalæmon.
Their most active and obvious enemy is a plant, not an animal,—to wit, a filamentous alga that blocks up their canals by its rapid growth (p. 79).
(b)Beneficial Organisms.
The most abundant and possibly the most important organisms that may be considered as benefactors to the Spongillidæ are the green corpuscles that live in the cells of certain species (fig. 2, p. 31), notablySpongilla lacustris,S. proliferens, andDosilia plumosa. I have already said that these bodies are in all probability algæ which live free in the water and move actively at one stage of their existence, but some of them are handed on directly from a sponge to its descendants in the cells of the gemmule. In their quiescent stage they have been studied by several zoologists, notably by Sir Ray Lankester[Q]and Dr. W. Weltner[R], but the strongest light that has been cast on their origin is given by the researches of Dr. F. W. Gamble and Mr. F. Keeble (Q. J. Microsc. Sci. London, xlvii, p. 363, 1904, and li, p. 167, 1907). These researches do not refer directly to the Spongillidæ but to a little flat-worm that lives in the sea,Convoluta roscoffiensis. The green corpuscles of this worm so closely resemble those ofSpongillathat we are justified in supposing a similarity of origin. It has been shown by the authors cited that the green corpuscles of the worm are at one stage minute free-living organisms provided at one end with four flagella and at the other with a red pigment spot. The investigators are of the opinion that these organisms exhibitthe essential characters of the algæ known as Chlamydomonadæ, and that after they have entered the worm they play for it the part of an excretory system.
As they exist in the cells ofSpongillathe corpuscles are minute oval bodies of a bright green colour and each containing a highly refractile colourless granule. A considerable number may be present in a single cell. It is found in European sponges that they lose their green colour if the sponge is not exposed to bright sunlight. In India, however, where the light is stronger, this is not always the case. Even when the colour goes, the corpuscles can still be distinguished as pale images of their green embodiment. They are calledChlorellaby botanists, who have studied their life-history but have not yet discovered the full cycle. See Beyerinck in the Botan. Zeitung for 1890 (vol. xlviii, p. 730, pl. vii; Leipzig), and for further references West's 'British Freshwater Algæ,' p. 230 (1904).
The list of beneficent organisms less commonly present than the green corpuscles includes aChironomuslarva that builds parchment-like tubes in the substance ofSpongilla carteriand so assists in supporting the sponge, and of a peculiar little worm (Chætogaster spongillæ[S]) that appears to assist in cleaning up the skeleton of the same sponge at the approach of the hot weather and in setting free the gemmules (p. 93).
(c)Organisms that take shelter in the Sponge or adhere to it externally.
There are many animals which take shelter in the cavities of the sponge without apparently assisting it in any way. Among these are the little fishGobius alcockii, which lays its eggs inside the oscula ofS. carteri, thus ensuring not only protection but also a proper supply of oxygen for them (p. 94); the molluscs (Corbula, spp.) found insideS. albavar.bengalensis(p. 78); and the Isopod (Tachæa spongillicola) that makes its way into the oscula ofSpongilla carteriandS. crateriformis(pp. 86, 94).
In Europe a peculiar ciliated Protozoon (Trichodina spongillæ) is found attached to the external surface of freshwater sponges. I have noticed a similar species at Igatpuri onSpongilla crateriformis, but it has not yet been identified. It probably has no effect, good or bad, on the sponge.
Freshwater Sponges in relation to Man.
In dealing withSpongilla carteriI have suggested that sponges may be of some hygienic importance in absorbing putrid organic matter from water used both for ablutionary and for drinking purposes, as is so commonly the case with regard to ponds in India. Their bad odour has caused some species of Spongillidæto be regarded as capable of polluting water, but a mere bad odour does not necessarily imply that they are insanitary.
Unless my suggestion that sponges purify water used for drinking purposes by absorbing putrid matter should prove to be supported by fact, the Spongillidæ cannot be said to be of any practical benefit to man. The only harm that has been imputed to them is that of polluting water[T], of blocking up water-pipes by their growth—a very rare occurrence,—and of causing irritation to the human skin by means of their spicules—a still rarer one. At least one instance is, however, reported in which men digging in a place where a pond had once been were attacked by a troublesome rash probably due to the presence of sponge-spicules in the earth, and students of the freshwater sponges should be careful not to rub their eyes after handling dried specimens.
Indian Spongillidæ Compared With Those of Other Countries.
In Weltner's catalogue of the freshwater sponges (1895) seventy-six recent species of Spongillidæ (excludingLubosmirskia) are enumerated, and the number now known is well over a hundred. In India we have twenty-nine species, subspecies, and varieties, while from the whole of Europe only about a dozen are known. In the neighbourhood of Calcutta nine species, representing three genera and a subgenus, have been found; all of them occur in the Museum tank. The only other region of similar extent that can compare with India as regards the richness of its freshwater sponge fauna is that of the Amazon, from which about twenty species are known. From the whole of North America, which has probably been better explored than any other continent so far as Spongillidæ are concerned, only twenty-seven or twenty-eight species have been recorded.
The Indian species fall into seven genera, one of which (Spongilla) consists of three subgenera. With one exception (that ofPectispongilla, which has only been found in Southern India) these genera have a wide distribution over the earth's surface, and this is also the case as regards the subgenera of Spongilla. Four genera (Heteromeyenia,Acalle,Parmula, andUruguaya) that have not yet been found in India are known to exist elsewhere.
Five of the Indian species are known to occur in Europe, viz.,Spongilla lacustris,S. crateriformis,S. carteri,S. fragilis,Trochospongilla pennsylvanica; whileEphydatia meyeniis intermediate between the two commonest representatives of its genus in the Holarctic Zone,Ephydatia fluviatilisandE. mülleri. Of the species that occur both in India and in Europe, two (SpongillalacustrisandS. fragilis) are found in this country in forms sufficiently distinct to be regarded as subspecies or local races. Perhaps this course should also be taken as regards the Indian forms ofS. carteri, of which, however, the commonest of the Indian races would be the typical one; butS. crateriformisandT. pennsylvanicaseem to preserve their specific characters free from modification, whether they are found in Europe, Asia, or America.
The freshwater sponges of Africa have been comparatively little studied, but two Indian species have been discovered,S. bombayensisin Natal andS. albavar.cerebellatain Egypt. Several of the species from the Malabar Zone are, moreover, closely allied to African forms (p. 11).
Fossil Spongillidæ.
The Spongillidæ are an ancient family. Young described a species (Spongilla purbeckensis) from the Upper Jurassic of Dorset (Geol. Mag. London (new series) v, p. 220 (1878)), while spicules, assigned by Ehrenberg to various genera but actually those ofSpongilla lacustrisor allied forms, have been found in the Miocene of Bohemia (see Ehrenberg's 'Atlas für Micro-Geologie,' pl. xi (Leipzig, 1854), and Traxler in Földt. Közl., Budapest, 1895, p. 211).Ephydatiais also known in a fossil condition, but is probably less ancient thanSpongilla.
Ehrenberg found many sponge spicules in earth from various parts of the Indian Empire (including Baluchistan, Mangalore, Calcutta, the Nicobars and Nepal) and elsewhere, and it might be possible to guess at the identity of some of the more conspicuous species figured in his 'Atlas.' The identification of sponges from isolated spicules is, however, always a matter of doubt, and in some cases Ehrenberg probably assigned spicules belonging to entirely different families or even orders to the same genus, while he frequently attributed the different spicules of the same species to different genera. Among his fossil (or supposed fossil) genera that may be assigned to the Spongillidæ wholly or in part areAphidiscus,Spongolithis,LithastericusandLithosphæridium, many of the species of these "genera" certainly belonging toSpongillaandEphydatia.
Oriental Spongillidæ not yet found in India.
Few freshwater sponges that have not been found in India are as yet known from the Oriental Region, and there is positive as well as negative evidence that Spongillidæ are less abundant in Malaysia than in this country. The following list includes the names of those that have been found, with notes regarding each species. It is quite possible that any one of them may be found at any time within the geographical boundaries laid down for this 'Fauna.' I have examined types or co-types in all cases except that ofEphydatia fortis, Weltner.
I.Spongilla(Euspongilla)microsclerifera*, Annandale (Philippines). P. U.S. Mus. xxxvii, p. 131 (1909).
This sponge is closely related toS. lacustris, but apparently does not produce branches. It is remarkable for the enormous number of microscleres in its parenchyma.
II.S.(Euspongilla)philippinensis*, Annandale (Philippines). P. U.S. Mus. xxxvi, p. 629 (1909).
Related toS. albaand still more closely toS. sceptrioidesof Australia. From the former it is readily distinguished by having minutely spined megascleres, green corpuscles, slender gemmule-spicules with short spines and no free microscleres.
III.S.(?Euspongilla)yunnanensis*, Annandale (W. China). Rec. Ind. Mus. v, p. 197 (1910).
Apparently allied toS. philippinensisbut with smooth skeleton-spicules and a more delicate skeleton.
IV.S.(Stratospongilla)sinensis*, Annandale (Foochow, China). P. U.S. Mus. xxxviii, p. 183 (1910).
This species andS. clementisare referred toStratospongillawith some doubt. Their gemmules are intermediate in structure between those of that subgenus and those ofEuspongilla. InS. sinensisthe gemmules are packed together in groups at the base of the sponge, and their spicules are smooth, stout, and gradually pointed.
V.S.(Stratospongilla)clementis*, Annandale (Philippines). P. U.S. Mus. xxxvi, p. 631 (1909).
The gemmules are single and closely adherent at the base of the sponge. Their spicules are very slender and minutely spined.
VI.S.(?Stratospongilla)coggini*, Annandale (W. China). Rec. Ind. Mus. v, p. 198 (1910).
The gemmules apparently lack microscleres. They resemble those ofS. clementis, to which the species is probably related, in other respects. The skeleton-spicules are spiny and rather stout, the species being strongly developed at the two ends.
VII.S.(Stratospongilla)sumatrana*, Weber (Malay Archipelago). Zool. Ergebnisse einer Reise in Niederländisch Ost-Indien, i. p. 38 (1890).
Closely allied toS. indica(p. 100) but with pointed skeleton-spicules.
VIII.Ephydatia fortis, Weltner (Philippines). Arch. Naturgesch. lxi(i), p. 141 (1895).
This species is remarkable for the great development of the spines on the shaft of the gemmule-spicules.
IX.Ephydatia bogorensis*, Weber (Malay Archipelago). Zool. Ergebnisse einer Reise in Niederländisch Ost-Indien, i, p. 33 (1890).
The gemmule-spicules have rather narrow flattish disks, the edge of which is feebly but closely serrated.
X.E. blembingia*, Evans (Malay Peninsula). Q. J. Microsc. Sci. London, xliv, p. 81 (1901).
The gemmules resemble those ofDosilia plumosabut are spherical. There are no free microscleres.
XI.Tubella vesparium*, v. Martens (Borneo). Arch. Naturg. Berlin, xxxiv, p. 62 (1868).
Closely related toT. vesparioides(p. 189), but with spiny megascleres.
As regardsSpongilla decipiens*, Weber, from the Malay Archipelago, see p. 97.
II.
History of the Study of Freshwater Sponges.
The bath-sponge was known to the Greeks at an early date, and Homer refers to it as being used for cleansing furniture, for expunging writing, and for ablutionary purposes. He also mentions its peculiar structure, "with many holes." "Many things besides," wrote the English naturalist Ray in his 'Historia Plantarum' (1686), "regarding the powers and uses of sponges have the Ancients: to them refer." Ray himself describes at least one freshwater species, which had been found in an English river, and refers to what may be another as having been brought from America. In the eighteenth century Linné, Pallas and other authors described the commoner European Spongillidæ in general terms, sometimes as plants and sometimes as animals, more usually as zoophytes or "plant-animals" partaking of the nature of both kingdoms. The gemmules were noted and referred to as seeds. The early naturalists of the Linnæan Epoch, however, added little to the general knowledge of the Spongillidæ, being occupied with theory in which theological disputes were involved rather than actual observation, and, notwithstanding the fact that the animal nature of sponges was clearly demonstrated by Ellis[U]in 1765, it was not until the nineteenth century was well advanced that zoologists could regard sponges in anything like an impartial manner.
One of the pioneers in the scientific study of the freshwaterforms was the late Dr. H. J. Carter, who commenced his investigations, and carried out a great part of them, in Bombay with little of the apparatus now considered necessary, and with a microscope that must have been grossly defective according to modern ideas. His long series of papers (1848-1887) published in the 'Annals and Magazine of Natural History' is an enduring monument to Indian zoology, and forms the best possible introduction to the study of the Spongillidæ. Even his earlier mistakes are instructive, for they are due not so much to actual errors in observation as to a faithful transcription of what was observed with faulty apparatus.
Contemporary with Carter were two authors whose monographs on the freshwater sponges did much to advance the study of the group, namely, J. S. Bowerbank, whose account of the species known at the time was published in the 'Proceedings of the Zoological Society of London' in 1882, and the veteran American naturalist Mr. Edward Potts, whose study of the freshwater sponges culminated in his monograph published in the 'Proceedings of the Academy of Natural Sciences of Philadelphia' in 1887. Carter's own revision of the group was published in the 'Annals and Magazine of Natural History' in 1881. The names of Vejdovsky, who prefaced Potts's monograph with an account of the European species, and of Dybowsky, who published several important papers on classification, should also be mentioned, while Weltner's catalogue of the known species (1895) is of the greatest possible value to students of the group.
Many authors have dealt with the physiology, reproduction and development of the Spongillidæ, especially in recent years; Dr. R. Evans's description of the larva ofSpongilla lacustris(1899), and his account of the development of the gemmule inEphydatia blembingia(1901), Zykoff's account of the development of the gemmule and of the sponge from the gemmule (1892), and Weltner's observations on colour and other points (1893, 1907), may be mentioned in particular. Laurent's observations on development (1844), which were published in the 'Voyage de la Bonite,' and especially the exquisite plates which accompany them, have not received the notice they deserve, probably on account of their method of publication.
Literature.
The fullest account of the literature on the Spongillidæ as yet published will be found in the first of Weltner's 'Spongillidenstudien' (Archiv für Naturgeschichte, lix (i), p. 209, 1893). Unfortunately it contains no references of later date than 1892. The following list is not a complete bibliography, but merely a list of books and papers that should prove of use to students of the Oriental Spongillidæ.