[Footnote K: Except in "_Proterospongia_," an organism of doubtful affinities but not a sponge. It consists of a ma.s.s 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 h.o.r.n.y 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 h.o.r.n.y 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 ca.n.a.ls 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 cla.s.s Demospongiae, 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 cla.s.s, order, family, genus, and species of the sponge. Formerly sponges were separated into calcareous, siliceous, and h.o.r.n.y sponges by the nature of their skeleton; and although the system of cla.s.sification 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 Demospongiae that have spicules at all have them of the latter substance, and the grade Monaxonida, in which the freshwater sponges const.i.tute the family Spongillidae, 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 Spongillidae 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 a.s.sert, 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 cla.s.sification of the Spongillidae. 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 circ.u.mstances.

GENERAL STRUCTURE OF THE SPONGILLIDae.

It would be impracticable in this introduction to give a full account of the structure of the Spongillidae, 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.

[Ill.u.s.tration: Fig. 1.--Diagram of a vertical section through a freshwater sponge (_modified from Kukenthal_).

A=pores; B=subdermal cavity; C=inhalent ca.n.a.l; D=ciliated chamber; E=exhalent ca.n.a.l; 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 Demospongiae have several or many oscula, but even in the Spongillidae 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 pa.s.s 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 s.p.a.ce differs greatly in extent in different species. From the subdermal s.p.a.ce the water is forced by the action of the flagella into narrow tubular ca.n.a.ls that carry it into the ciliated chambers. Thence it pa.s.ses into other ca.n.a.ls, 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.

[Ill.u.s.tration: Fig. 2.--Sponge cells.

A=bubble-cells of _Ephydatia mulleri_, 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 aegagrophila_, 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 Demospongiae, 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 _Spongillae_.

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 s.p.a.ces between adjacent cells, which may be a little more crowded together than is usual. But in others (e. g., _Spongilla proliferens_ and _Spongilla cra.s.sissima_) 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 ca.n.a.ls 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 Spongillidae 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 a.s.sume 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.

[Ill.u.s.tration: Fig. 3.--Radial sections of fragments of the skeletons of _Spongillae_.

A, _S. cra.s.sissima_ var. _cra.s.sior_ (from Rajshahi); B, _S. carteri_ (from Calcutta); _a_=transverse, _b_=radiating fibres; _e_=external surface of the sponge.]

In the Spongillidae 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 ma.s.s 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 a.s.sume in outline the form of an hour-gla.s.s (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.

[Ill.u.s.tration: Fig. 4.--Part of an oscular collar of _Spongilla lacustris_ subsp. _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 Spongillidae, 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 Spongillidae 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 cra.s.sissima_) its termination is marked at both ends of the megasclere by a minute conical protuberance in the silica.

Unless sponges are alchemists and can trans.m.u.te 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 pa.s.s 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 a.s.sist 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 Spongillidae, 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 ill.u.s.trate the absorption and final elimination of solid colouring matter by placing a living sponge (small specimens of _Spongilla carteri_ are suitable) in a gla.s.s 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 Spongillidae 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 ca.n.a.ls and substance of the sponge of parasitic algaae, 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.

[Ill.u.s.tration: Fig. 5.--Part of a type-specimen of _Spongilla lacustris_ subsp. _reticulata_ (nat. size).]