The external form of sponges is very variable, but each species, subspecies, or variety of the Spongillidae has normally a characteristic appearance. The European race of _Spongilla lacustris_, for example, consists in favourable circ.u.mstances 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 ma.s.ses. 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 ma.s.ses 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 ma.s.ses, 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 cra.s.sissima_) naturally a.s.sume 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.

[Ill.u.s.tration: Fig. 6.--Radial section through part of a dried sponge of _Spongilla cra.s.sissima_ (from Calcutta), 5.]

Spongillidae differ greatly in consistency. _Spongilla cra.s.sissima_ 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 a.s.sociated with them. In _Spongilla cra.s.sissima_, 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 c.o.c.ks...o...b... 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 ma.s.s 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 ill.u.s.trates an excellent example of variation in external form to which it is impossible to a.s.sign 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 Spongillidae 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 ca.n.a.ls 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 pa.s.s 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 Spongillidae 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 Spongillidae 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_), s.e.xual 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 sh.e.l.l, 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 a.s.sume 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 ma.s.s of spermatozoa surrounded by a single covering cell, which they finally rupture, and so escape.

[Ill.u.s.tration: Fig. 7.--Diagram of a vertical section through the gemmule of _Spongilla 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 as.e.xual reproductive bodies peculiar to the sponges, but not to the Spongillidae. They resemble the statoblasts of the phylactolaematous 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 ma.s.s 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 s.p.a.ces 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 j.a.pan 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 Spongillidae 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-s.p.a.ces 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 ma.s.s of chitin (or rather spongin), while the cells in a cellular one are only larger and more regular air-s.p.a.ces with thin polygonal walls and flat horizontal part.i.tions. The walls of these s.p.a.ces 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 Spongillidae, 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. Se. 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 ca.n.a.ls, 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 ca.n.a.ls 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 ma.s.ses 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).