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 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 (_Chætogaster spongillæ_) also found among the zooecia 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.