In the ponds of Calcutta a large number of animals are found associated 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 circumstances 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 canals of S. carteri, its proboscis inserted into the substance of the sponge. If the sponge dies and the water becomes foul the larvæ 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 cocoon on its surface. Hence they emerge as perfect insects after about a week.
An animal that may be an enemy of S. carteri is a flat-worm (an undescribed species of Planaria) common in its larger canals 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 Palæmon 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 larvæ 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 zoœcia 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 œconomy of the sponge is a peculiar little worm (Chætogaster spongillæ) also found among the zoœcia of Plumatella and belonging to a widely distributed genus of which several species are found in association with pond-snails. Chætogaster spongillæ often occurs in enormous numbers in dead or dying sponges of S. carteri, apparently feeding on the decaying organic matter of the sponge and assisting by its movements in releasing numerous gemmules. In so doing it undoubtedly assists in the dissemination of the species.
Major J. Stephenson (Rec. Ind. Mus. v, p. 233) has recently found two other species of oligochætes inhabiting S. carteri var. lobosa from Travancore. Both these species, unlike Chætogaster spongillæ, 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. cæca) has only been found in this situation, but the other (var. punjabensis) lives free as well as in association with the sponge, in which the blind form was the commoner of the two.
The majority of the animals found in association with S. carteri gain shelter without evident assistance to the sponge. This is the case as regards the little fish (Gobius alcockii), 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 passes 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 Tachæa spongillicola[[AF]], the adults of which are found in the canal of this and other sponges, while the young cling to the external surface of the carapace of Palæmon lamarrei and other small prawns. Many worms and insects of different kinds also enter the canals 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.
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 algæ are found associated 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 massive 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 massive, growing in the same ponds as S. carteri.