A large proportion, however, still remain under water; but even these begin to show signs of decay at this period. Their cells migrate to the extremities of the sponge, leaving a ma.s.s of gemmules in the centre, and finally perish.

Few sponges exist in an active condition throughout the hot weather. The majority of those that do so exhibit a curious phenomenon. Their surface becomes smoothly rounded and they have a slightly pinkish colour; the majority of the cells of their parenchyma, if viewed under a high power of the microscope, can be seen to be gorged with very minute drops of liquid. This liquid is colourless in its natural condition, but if the sponge is plunged into alcohol the liquid turns of a dark brown colour which stains both the alcohol and the sponge almost instantaneously.

Probably the liquid represents some kind of reserve food-material. Even in the hot weather a few living sponges of the species may be found that have not this peculiarity, but, in some ponds at any rate, the majority that survive a.s.sume the peculiar summer form, which I have also found at Lucknow.

Reproduction takes place in _S. carteri_ in three distinct ways, two of which may be regarded as normal, while the third is apparently the result of accident. If a healthy sponge is torn into small pieces and these pieces are kept in a bowl of water, little ma.s.ses of cells congregate at the tips of the radiating fibres of the skeleton and a.s.sume a globular form. At first these cells are h.o.m.ogeneous, having clear protoplasm full of minute globules of liquid. The ma.s.ses differ considerably in size but never exceed a few millimetres in diameter. In about two days differentiation commences among the cells; then spicules are secreted, a central cavity and an external membrane formed, and an aperture, the first osculum, appears in the membrane. In about ten days a complete young sponge is produced, but the details of development have not been worked out.

The most common normal form of reproduction is by means of gemmules, which are produced in great numbers towards the end of the cold weather.

If small sponges are kept alive in an aquarium even at the beginning of the cold weather, they begin to produce gemmules almost immediately, but these gemmules although otherwise perfect, possess few or no gemmule-spicules. If the sponge becomes desiccated at the end of the cold weather and is protected in a sheltered place, some or all of the gemmules contained in the meshes of its skeleton germinate _in situ_ as soon as the water reaches it again during the "rains." It is by a continuous or rather periodical growth of this kind, rea.s.sumed season after season, that large ma.s.ses of sponge are formed. In such ma.s.ses it is often possible to distinguish the growth of the several years, but as a rule the layers become more or less intimately fused together, for no limiting membrane separates them. A large proportion of the gemmules are, however, set free and either float on the surface of the water that remains in the ponds or are dried up and carried about by the wind. In these circ.u.mstances they do not germinate until the succeeding cold weather, even if circ.u.mstances other than temperature are favourable; but as soon as the cold weather commences they begin to produce new sponges with great energy.

s.e.xual reproduction, the second normal form, takes place in _S. carteri_ mainly if not only at the approach of a change of season, that is to say about March, just before the hot weather commences, and about November, just as the average temperature begins to sink to a temperate level. At these seasons healthy sponges may often be found full of eggs and embryos, which lie in the natural cavities of the sponge without protecting membrane.

In the ponds of Calcutta a large number of animals are found a.s.sociated in a more or less definite manner with _Spongilla carteri_. Only one, however, can be described with any degree of certainty as being in normal circ.u.mstances an enemy, namely the larva of _Sisyra indica_,[AD]

and even in the case of this little insect it is doubtful how far its attacks are actually injurious to the sponge. The larva is often found in considerable numbers clinging to the oscula and wide efferent ca.n.a.ls of _S. carteri_, its proboscis inserted into the substance of the sponge. If the sponge dies and the water becomes foul the larvae swim or crawl away. If the sponge dries up, they leave its interior (in which, however, they sometimes remain for some days after it has become dry) and pupate in a silken coc.o.o.n on its surface. Hence they emerge as perfect insects after about a week.

[Footnote AD: Needham. Rec. Ind. Mus. iii, p. 206 (1909).]

An animal that may be an enemy of _S. carteri_ is a flat-worm (an undescribed species of _Planaria_) common in its larger ca.n.a.ls and remarkable for the small size of its pharynx. The same worm, however, is also found at the base of the leaves of bulrushes and in other like situations, and there is no evidence that it actually feeds on the sponge. Injured sponges are eaten by the prawn _Palaemon lamarrei_, which, however, only attacks them when the dermal membrane is broken. A _Tanypus_ larva (Chironomid Diptera) that makes its way though the substance of the sponge may also be an enemy; it is commoner in decaying than in vigorous sponges.

The presence of another Chironomid larva (_Chironomus_, sp.) appears to be actually beneficial. In many cases it is clear that this larva and the sponge grow up together, and the larva is commoner in vigorous than in decayed sponges. Unlike the _Tanypus_ larva, it builds parchment-like tubes, in which it lives, on the surface of the sponge. The sponge, however, often grows very rapidly and the larva is soon in danger of being engulfed in its substance. The tube is therefore lengthened in a vertical direction to prevent this catastrophe and to maintain communication with the exterior. The process may continue until it is over an inch in length, the older part becoming closed up owing to the pressure of the growing sponge that surrounds it. Should the sponge die, the larva lives on in its tubes without suffering, and the ends of tubes containing larvae may sometimes be found projecting from the worn surface of dead sponges. The larva does not eat the sponge but captures small insects by means of a pair of legs on the first segment of its thorax.

In so doing it thrusts the anterior part of its body out of the tube, to the inner surface of which it adheres by means of the pair of false legs at the tip of the abdomen. This insect, which is usually found in the variety _mollis_, appears to do good to the sponge in two ways--by capturing other insects that might injure it and by giving support to its very feeble skeleton.

A precisely similar function, so far as the support of the sponge is concerned, is fulfilled by the tubular zooecia of a phase of the polyzoon _Plumatella fruticosa_ (see p. 218) which in India is more commonly found embedded in the substance of _S. carteri_ than in that of any other species, although in Great Britain it is generally found in that of _S. lacustris_, which is there the commonest species of freshwater sponge.

Another animal that appears to play an active part in the oeconomy of the sponge is a peculiar little worm (_Chaetogaster spongillae_) also found among the zooecia of _Plumatella_ and belonging to a widely distributed genus of which several species are found in a.s.sociation with pond-snails. _Chaetogaster spongillae_ often occurs in enormous numbers in dead or dying sponges of _S. carteri_, apparently feeding on the decaying organic matter of the sponge and a.s.sisting by its movements in releasing numerous gemmules. In so doing it undoubtedly a.s.sists in the dissemination of the species.

Major J. Stephenson (Rec. Ind. Mus. v, p. 233) has recently found two other species of oligochaetes inhabiting _S. carteri_ var. _lobosa_ from Travancore. Both these species, unlike _Chaetogaster spongillae_, belong to a genus that is vegetarian in habits. One of them, _Nais pectinata_, has not yet been found elsewhere, while the other, _Nais communis_, has a very wide distribution. The latter, however, occurs in the sponge in two forms--one with eyes, the other totally blind. The blind form (_N.

communis_ var. _caeca_) has only been found in this situation, but the other (var. _punjabensis_) lives free as well as in a.s.sociation with the sponge, in which the blind form was the commoner of the two.

The majority of the animals found in a.s.sociation with _S. carteri_ gain shelter without evident a.s.sistance to the sponge. This is the case as regards the little fish (_Gobius alc.o.c.kii_), one of the smallest of the vertebrates (length about 1/2 inch), which lays its eggs in the patent oscula, thus securing for them a situation peculiarly favourable to their development owing to the constant current of water that pa.s.ses over them. In the absence of sponges, however, this fish attaches its eggs to the floating roots of the water-plant _Pistia stratiotes_.

Numerous small crustacea[AE] also take temporary or permanent refuge in the cavities of _S. carteri_, the most noteworthy among them being the Isopod _Tachaea spongillicola_[AF], the adults of which are found in the ca.n.a.l of this and other sponges, while the young cling to the external surface of the carapace of _Palaemon lamarrei_ and other small prawns.

Many worms and insects of different kinds also enter the ca.n.a.ls of _S.

carteri_, especially when the sponge is becoming desiccated; from half-dry sponges numerous beetles and flies may be bred, notably the moth-fly _Psychoda nigripennis_[AG] of which enormous numbers sometimes hatch out from such sponges.

[Footnote AE: According to the late Rai Bahadur R. B.

Sanyal, freshwater sponges are called in Bengali "shrimps'

nests." From his description it is evident that he refers mainly to _S. carteri_ (see Hours with Nature, p. 46; Calcutta 1896).]

[Footnote AF: Stebbing, J. Linn. Soc. x.x.x, p. 40; Annandale, Rec. Ind. Mus. i, p. 279.]

[Footnote AG: Brunetti, Rec. Ind. Mus. ii, p. 376 (1908).]

As the sponge grows it frequently attaches itself to small molluscs such as the young of _Vivipara bengalensis_, which finally become buried in its substance and thus perish. Possibly their decaying bodies may afford it nourishment, but of the natural food of sponges we know little. _S.

carteri_ flourishes best and reaches its largest size in ponds used for domestic purposes by natives of India, and thrives in water thick with soap-suds. It is possible, though direct proof is lacking, that the sponge does good in purifying water used for washing the clothes, utensils, and persons of those who drink the same water, by absorbing decaying animal and vegetable matter from it.

Various minute algae are found a.s.sociated with _S. carteri_, but of these little is yet known. The green flush sometimes seen on the surface of the typical form is due to the fact that the superficial cells of the parenchyma contain green corpuscles. These, however, are never very numerous and are not found in the inner parts of the sponge, perhaps owing to its ma.s.sive form. It is noteworthy that these green bodies flourish in large numbers throughout the substance of sponges of _S.

proliferens_, a species always far from ma.s.sive, growing in the same ponds as _S. carteri_.

9. Spongilla fragilis, _Leidy_.

_Spongilla fragilis_, Leidy, P. Ac. Philad. 1851, p. 278.

_Spongilla lordii_, Bowerbank, P. Zool. Soc. London, 1863, p. 466, pl. x.x.xviii, fig. 17.

_Spongilla contecta_, Noll, Zool. Garten*, 1870, p. 173.

_Spongilla ottavaensis_, Dawson, Canad. Nat.* (new series) viii, p. 5 (1878).

_Spongilla sibirica_, Dybowski, Zool. Anz., Jahr. i, p. 53 (1878).

_Spongilla morgiana_, Potts, P. Ac. Philad. 1880, p. 330.

_Spongilla lordii_, Carter, Ann. Nat. Hist. (5) vii, p. 89, pl. vi, fig. 13 (1881).

_Spongilla sibirica_, Dybowski, Mem. Ac. St. Petersb. (7) x.x.x, no. x, p. 10, fig. 12.

_Spongilla glomerata_, Noll, Zool. Anz., Jahr. ix, p. 682 (1886).

_Spongilla fragilis_, Vejdovsky, P. Ac. Philad. 1887, p.

176.

_Spongilla fragilis_, Potts, _ibid._ p. 197, pl. v, fig. 2; pl. viii, figs. 1-4.

_Spongilla fragilis_, Weltner, Arch. Naturg. lix (1), p.

266, pl. ix, figs. 18-20 (1893).

_Spongilla fragilis_, _id._, Arch. Naturg. lxi (i), p. 117 (1895).

_Spongilla fragilis_, _id._, in Semon's Zool. Forsch. in Austral. u. d. Malay. Arch. v, part v, p. 523.

_Spongilla fragilis_, Annandale, P. U.S. Mus. x.x.xvii, p. 402 (1909).

_Spongilla fragilis_, _id._, Annot. Zool. j.a.pon. vii, part ii, p. 106, pl. ii, fig. 1 (1909).

_Sponge_ flat, lichenoid, never of great thickness, devoid of branches, dense in texture but very friable; colour brown, green, or whitish; oscula numerous, small, flat, distinctly star-shaped.

_Skeleton_ with well defined radiating and transverse fibres, which are never strong but form a fairly dense network with a small amount of spongin.

_Spicules._ Skeleton-spicules smooth, sharply pointed, moderately stout, as a rule nearly straight. No flesh-spicules. Gemmule-spicules cylindrical, blunt or abruptly pointed, nearly straight, covered with relatively stout, straight, irregular spines, which are equally distributed all over the spicule.

_Gemmules_ bound together in free groups of varying numbers and forming a flat layer at the base of the sponge; each gemmule small in size, surrounded by a thick cellular coat of several layers; with a relatively long and stout foraminal tubule, which projects outwards through the cellular coat at the sides of the group or at the top of the basal layer of gemmules, is usually curved, and is not thickened at the tip; more than one foraminal tubule sometimes present on a single gemmule; gemmule-spicules arranged horizontally or at the base of the cellular coat.

The species as a species is easily distinguished from all others, its nearest ally being the N. American _S. ingloriformis_ with spa.r.s.ely spined skeleton-spicules which are very few in number, and gemmule groups in which the foraminal tubules all open downwards.

Several varieties of _S. fragilis_ have been described in Europe and America.

TYPE.--Potts refers to the type as being in the Academy of Natural Sciences at Philadelphia.

GEOGRAPHICAL DISTRIBUTION.--All over Europe and N. America; also in Siberia, Australia, and S. America. The species is included in this work in order that its Asiatic local races may be fitly described.