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.
The most abundant and possibly the most important organisms that may be considered as benefactors to the Spongillidæ are the green corpuscles that live in the cells of certain species (fig. 2, p. 31), notably Spongilla lacustris, S. proliferens, and Dosilia plumosa. I have already said that these bodies are in all probability algæ which live free in the water and move actively at one stage of their existence, but some of them are handed on directly from a sponge to its descendants in the cells of the gemmule. In their quiescent stage they have been studied by several zoologists, notably by Sir Ray Lankester[[Q]] and Dr. W. Weltner[[R]], but the strongest light that has been cast on their origin is given by the researches of Dr. F. W. Gamble and Mr. F. Keeble (Q. J. Microsc. Sci. London, xlvii, p. 363, 1904, and li, p. 167, 1907). These researches do not refer directly to the Spongillidæ but to a little flat-worm that lives in the sea, Convoluta roscoffiensis. The green corpuscles of this worm so closely resemble those of Spongilla that we are justified in supposing a similarity of origin. It has been shown by the authors cited that the green corpuscles of the worm are at one stage minute free-living organisms provided at one end with four flagella and at the other with a red pigment spot. The investigators are of the opinion that these organisms exhibit the essential characters of the algæ known as Chlamydomonadæ, and that after they have entered the worm they play for it the part of an excretory system.