Chapter V: Introduction: To Part I (1)

I.

THE PHYLUM PORIFERA.

The phylum Porifera or Spongiæ includes the simplest of the Metazoa or multicellular animals. From the compound Protozoa its members are distinguished by the fact that the cells of which they are composed exhibit considerable differentiation both in structure and in function, and are associated together in a definite manner, although they are not combined to form organs and systems of organs as in the higher Metazoa. Digestion, for instance, is performed in the sponges entirely by individual cells, into the substance of which the food is taken, and the products of digestion are handed on to other cells without the intervention of an alimentary canal or a vascular system, while there is no structure in any way comparable to the nervous system of more highly organized animals.

The simplest form of sponge, which is known as an olynthus, is a hollow vase-like body fixed at one end to some solid object, and with an opening called the osculum at the other. The walls are perforated by small holes, the pores, from which the name Porifera is derived.

Externally the surface is protected by a delicate membrane formed of flattened cells and pierced by the pores, while the interior of the vase is covered with curious cells characteristic of the sponges, and known as choanocytes or collar-cells. They consist of minute oval or pear-shaped bodies, one end of which is provided with a rim or collar of apparently structureless membrane, while a flagellum or whip-like lash projects from the centre of the surface surrounded by the collar. These collar-cells are practically identical with those of which the Protozoa known as Choanoflagellata consist; but it is only in the sponges[K] that they are found constantly associated with other cells unlike themselves.

[Footnote K: Except in "_Proterospongia_," an organism of
doubtful affinities but not a sponge. It consists of a mass
of jelly containing ordinary cells, with collar-cells
_outside_.]

In addition to the collar-cells, which form what is called the gastral layer, and the external membrane (the derma or dermal membrane), the sponge contains cells of various kinds embedded in a structureless gelatinous substance, through which they have the power of free movement. Most of these cells have also the power of changing their form in an "amoeboid" manner; that is to say, by projecting and withdrawing from their margin mobile processes of a more or less finger-like form, but unstable in shape or direction. The protoplasm of which some of the cells are formed is granular, while that of others is clear and translucent. Some cells, which (for the time being at any rate) do not exhibit amoeboid movements, are glandular in function, while others again give rise in various ways to the bodies by means of which the sponge reproduces its kind. There is evidence, however, that any one kind of cell, even those of the membrane and the gastral layer, can change its function and its form in case of necessity.

Most sponges possess a supporting framework or skeleton. In some it is formed entirely of a horny substance called spongin (as in the bath-sponge), in others it consists of spicules of inorganic matter (either calcareous or siliceous) secreted by special cells, or of such spicules bound together by spongin. Extraneous objects, such as sand-grains, are frequently included in the skeleton. The spongin is secreted like the spicules by special cells, but its chemical structure is much more complicated than that of the spicules, and it is not secreted (at any rate in most cases) in such a way as to form bodies of a definite shape. In the so-called horny sponges it resembles the chitin in which insects and other arthropods are clothed.

* * * * *

In no adult sponge do the collar-cells completely cover the whole of the internal surface, the olynthus being a larval form, and by no means a common larval form. It is only found in certain sponges with calcareous spicules. As the structure of the sponge becomes more complicated the collar-cells are tucked away into special pockets or chambers known as ciliated chambers, and finally the approach to these chambers, both from the external surface and from the inner or gastral cavity, takes the form of narrow tubes or canals instead of mere pores. With further complexity the simple internal cavity tends to disappear, and the sponge proliferates in such a way that more than one osculum is formed. In the class Demospongiæ, to which the sponges described in this volume belong, the whole system is extremely complicated.

The skeleton of sponges, when it is not composed wholly of spongin, consists of, or at any rate contains, spicules that have a definite chemical composition and definite shapes in accordance with the class, order, family, genus, and species of the sponge. Formerly sponges were separated into calcareous, siliceous, and horny sponges by the nature of their skeleton; and although the system of classification now adopted has developed into a much more complex one and a few sponges are known that have both calcareous and siliceous spicules, the question whether the spicules are formed of salts of lime or of silica (strictly speaking of opal) is very important. All Demospongiæ that have spicules at all have them of the latter substance, and the grade Monaxonida, in which the freshwater sponges constitute the family Spongillidæ, is characterized by the possession of spicules that have typically the form of a needle pointed at both ends. Although spicules of this simple form may be absent in species that belong to the grade, the larger spicules, which are called megascleres, have not normally more than one main axis and are always more or less rod-like in outline. They are usually arranged so as to form a reticulate skeleton. Frequently, however, the megascleres or skeleton-spicules are not the only spicules present, for we find smaller spicules (microscleres) of one or more kinds lying loose in the substance of the sponge and in the external membrane, or, in the Spongillidæ only, forming a special armature for the reproductive bodies known as gemmules.

All sponges obtain their food in the same way, namely by means of the currents of water set up by the flagella of the collar-cells. These flagella, although apparently there is little concerted action among them, cause by their rapid movements changes of pressure in the water contained in the cavities of the sponge. The water from outside therefore flows in at the pores and finally makes its way out of the oscula. With the water minute particles of organic matter are brought into the sponge, the collar-cells of which, and probably other cells, have the power of selecting and engulfing suitable particles. Inside the cells these particles undergo certain chemical changes, and are at least partially digested. The resulting substances are then handed on directly to other cells, or, as some assert, are discharged into the common jelly, whence they are taken up by other cells.

Sponges reproduce their kind in more ways than one, _viz._, by means of eggs (which are fertilized as in other animals by spermatozoa), by means of buds, and by means of the peculiar bodies called gemmules the structure and origin of which is discussed below (p. 42). They are of great importance in the classification of the Spongillidæ. Sponges can also be propagated artificially by means of fission, and it is probable that this method of reproduction occurs accidentally, if not normally, in natural circumstances.

GENERAL STRUCTURE OF THE SPONGILLIDÆ.

It would be impracticable in this introduction to give a full account of the structure of the Spongillidæ, which in some respects is still imperfectly known. Students who desire further information should consult Professor Minchin's account of the sponges in Lankester's 'Treatise on Zoology,' part ii, or, if a less technical description is desired, Miss Sollas's contribution to the 'Cambridge Natural History,' vol. i, in which special attention is paid to _Spongilla_.

The diagram reproduced in fig. 1 gives a schematic view of a vertical section through a living freshwater sponge. Although it represents the structure of the organism as being very much simpler than is actually the case, and entirely omits the skeleton, it will be found useful as indicating the main features of the anatomy.

A=pores; B=subdermal cavity; C=inhalent canal; D=ciliated chamber; E=exhalent canal; F=osculum; G=dermal membrane; H=eggs; J=gemmule.]

It will be noted that the diagram represents an individual with a single osculum or exhalent aperture. As a rule adult Demospongiæ have several or many oscula, but even in the Spongillidæ sponges occur in which there is only one. New oscula are formed by a kind of proliferation that renders the structure still more complex than it is when only one exhalent aperture is present.

The little arrows in the figure indicate the direction of the currents of water that pass through the sponge. It enters through small holes in the derma into a subdermal cavity, which separates the membrane from the bulk of the sponge. This space differs greatly in extent in different species. From the subdermal space the water is forced by the action of the flagella into narrow tubular canals that carry it into the ciliated chambers. Thence it passes into other canals, which communicate with what remains of the central cavity, and so out of the oscula.

The ciliated chambers are very minute, and the collar-cells excessively so. It is very difficult to examine them owing to their small size and delicate structure. Fig. 2 D represents a collar-cell of a sponge seen under a very high power of the microscope in ideal conditions.

A=bubble-cells of _Ephydatia mülleri_, × 350 (_after Weltner_). B=gemmule-cell of _Spongilla lacustris_ containing green corpuscles (shaded dark), × 800 (_after Weltner_). C=gemmule-cell of _Ephydatia blembingia_ showing "tabloids" of food-material, × 1150 (_after Evans_). D=collar-cell of _Esperella ægagrophila_, × 1600 (_after Vosmaer and Pekelharing_). E=three stages in the development of a gemmule-spicule of _E. blembingia_ (_after Evans_), × 665. F=outline of porocytes of _S. proliferens_, × ca. 1290: _e_=dermal cell; _n_=nucleus; _p_=pore; _p.c._=pore-cell.]

The nature of the inhalent apertures in the external membrane has been much discussed as regards the Demospongiæ, but the truth seems to be that their structure differs considerably even in closely allied species. At any rate this is the case as regards the Indian _Spongillæ_. In all species the membrane is composed of flattened cells of irregular shape fitted together like the pieces of a puzzle-picture. In some species (e. g., _Spongilla carteri_) the apertures in the membrane consist merely of spaces between adjacent cells, which may be a little more crowded together than is usual. But in others (e. g., _Spongilla proliferens_ and _Spongilla crassissima_) in which the pores are extremely small, each pore normally pierces the middle of a flat, ring-shaped cell or porocyte. Occasionally, however, a pore may be found that is enclosed by two narrow, crescent-shaped cells joined together at their tips to form a ring. The porocytes of sponges like _Spongilla carteri_ are probably not actually missing, but instead of being in the external membrane are situated below the derma at the external entrance to the canals that carry water to the flagellated chambers or even at the entrance to the chambers themselves[L]. Some authors object on theoretical grounds to the statement that porocytes exist in the Demospongia, and it is possible that these cells have in this grade neither the same origin as, nor a precisely similar function to, the porocytes of other sponges. When they occur in the dermal membrane no great difficulty is experienced in seeing them under a sufficiently high power of the microscope, if the material is well preserved and mounted and stained in a suitable manner[M]. In most sponges the porocytes can contract in such a way that the aperture in their centre is practically closed, but this power appears to be possessed by the porocytes of _Spongilla_ only to a very limited extent, although they closely resemble the porocytes of other sponges in appearance.

[Footnote L: _Cf._ Weltner, "Spongillidenstudien, V," Arch.
Naturg. Berlin, lxxiii (i), p. 273 (1907).]

[Footnote M: It is difficult to see any trace of them in
thin microtome sections. A fragment of the membrane must be
mounted whole.]

The external membrane in many Spongillidæ is prolonged round and above the oscula so as to form an oscular collar. This structure is highly contractile, but cannot close together. As a rule it is much more conspicuous in living sponges than in preserved specimens.

It is not necessary to deal here with most of the cells that occur in the parenchyma or gelatinous part of the sponge. A full list of the kinds that are found is given by Dr. Weltner in his "Spongillidenstudien, V," p. 276 (Arch. Naturg. Berlin, lxxiii (i), 1907). One kind must, however, be briefly noticed as being of some systematic importance, namely the "bubble-cells" (fig. 2 A) that are characteristic of some species of _Ephydatia_ and other genera. These cells are comparatively large, spherical in form; each of them contains a globule of liquid which not only occupies the greater part of the cell, but forces the protoplasm to assume the form of a delicate film lining the cell-wall and covering the globule. In optical section "bubble-cells" have a certain resemblance to porocytes, but the cell is of course imperforate and not flattened.

SKELETON AND SPICULES.

A, _S. crassissima_ var. _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 than the 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.

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 as _Spongilla carteri_ in 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 sponge _Halichondria 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, but _Pectispongilla aurea_ is 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 of _Spongilla carteri_ are 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 of _Spongilla carteri_ these globules turn of a dark brown colour if treated with alcohol. The brilliant colour of _Pectispongilla aurea_ is 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 alba_ var. _bengalensis_). This cause is the growth in the canals and substance of the sponge of parasitic algaæ, 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.

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 of _Spongilla 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 bombayensis_ and _S. ultima_) never produce branches but always consist of lichenoid or cushion-shaped masses. The appearance of _Spongilla 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 as _Trochospongilla phillottiana_, cover large areas with a thin film of uniform thickness, while others (e. g., _Spongilla alba_ and _Ephydatia 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 as _Spongilla 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.

Spongillidæ differ greatly in consistency. _Spongilla crassissima_ and _Corvospongilla lapidosa_ are 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, as _Spongilla lacustris_, the variety _mollis_ of _S. carteri_, and _S. crateriformis_. The consistency of a sponge depends on two factors--the number of spicules present, and the amount of spongin. In _Corvospongilla lapidosa_ the number of spicules is very large indeed. They are not arranged so as to form a reticulate skeleton but interlock in all directions, and there is hardly any spongin associated with them. In _Spongilla 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. In _S. carteri_ var. _mollis_ both 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 of _Ephydatia_ which 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 of _Spongilla carteri_ that 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 of _Spongilla crateriformis_ taken 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 the environment. 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 of _Spongilla 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 which _Spongilla carteri_ is 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 lapidosa_ is 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 of _Spongilla lacustris_ fail 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 I in 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 of _Euspongilla_ and _Ephydatia_, 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 monoecious, 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 amoeboid 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 them in 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.

[Footnote N: Rec. Ind. Mus. i, p. 269 (1907).]

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 in _Spongilla_ from 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.

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 likely to 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 of _Ephydatia 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 gemmules of 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 of _Spongilla lacustris_ found in Lake Baa, Isle of Mull, as regards _S. helvetica_ from the Lake of Geneva, _S. moorei_ from Lake Tanganyika, and _S. coggini_ from Tali-Fu in Yunnan; also as regards the species of _Spongilla_ and _Ephydatia_ found in Lake Baikal, many of the sponges of which are said never to produce gemmules.

Except in the genus _Corvospongilla_ and the subgenus _Stratospongilla_, 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 in _Spongilla 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 in _Spongilla lacustris_ as 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 of _S. proliferens_, perhaps representing a form of fission rather than true budding (see 'Voyage de la Bonite: Zoophytologie,' Spongiaires, pl. i (Paris, 1844)).

[Footnote O: Proliferation whereby more than one osculum is
produced is really a form of budding, but in most sponges
this has become no longer a mode of reproduction but the
normal method by which size is increased, and must therefore
be considered merely as a vegetative process.]

In _Spongilla proliferens_, a common Indian species, the buds arise as 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 of _Spongilla_, but I have not been able to obtain evidence myself that this method of reproduction occurs normally in Indian species. In injured specimens of _Spongilla 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 in _S. lacustris_ were 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 of _Spongilla lacustris_ in 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 of _Spongilla alba_ var. _bengalensis_ treated 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 to _Ephydatia_ in 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 of _Spongilla proliferens_ grows 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. The skeleton-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.

[Footnote P: P. Ac. Philad. 1887, p. 162.]

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 of _Spongilla 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 is _Trochospongilla 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 that _Spongilla carteri_ has disappeared for some years from a small lake at Peradeniya in which it was formerly abundant, owing to the lake having been cleaned out, 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 of _Spongilla alba_ and _Ephydatia meyeni_) which in Calcutta are only found attached to such objects. The form of _S. 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 species _Corvospongilla lapidosa_ and our representatives of the subgenus _Stratospongilla_ are noteworthy. Some forms (e. g. _Spongilla carteri_ and _S. 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, but _Spongilla alba_ var. _bengalensis_ has been found in both, and is abundant in the latter; indeed, it has not been found in pure fresh water. _Spongilla travancorica_ has only been found in slightly brackish water, while _S. lacustris_ subsp. _reticulata_ and _Dosilia plumosa_ occur 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 (_Tanypus_ sp.) 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 genera _Limnæa_ and _Planorbis_, and prawns of the genus _Palæmon_.

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._