The Sponges of Lake Bhva. 25 



The relations between the sponge and the molluscs to which 

 it attaches itself are clearly those of parasite and host. The Lamel- 

 libranchs on which we observed the phenomenon most commonly 

 were Corbicula sandai and Nodularia reiniana. Both these species 

 live with the blunter anterior end of the shell buried in the sand 

 and the more pointed posterior extremity protruding therefrom. 

 It is naturally to the latter extremity that the sponge attaches itself. 

 Its growth is always at first restricted to a single valve and as long 

 as the mollusc can prevent it, by the currents of water it ejects 

 from time to time, from spreading to the other shell, it is unable 

 to injure the animal greatly, except in so far as it may impede its 

 movements. As the sponge is often several times as large as the 

 shell, it may possibly, however, cause inconvenience in this respect. 

 Sometimes, especially when the host is a relatively large Nodularia, 

 a cavity is formed near the base of the sponge where it arches 

 over the siphonal orifice of the mollusc, and Ave have observed 

 specimens in which this had occurred and in which larvae, doubtless 

 produced by the same sponge, had settled down and undergone 

 their metamorphosis on the other valve of the shell. Sooner or 

 later, at any rate in the case of Corbicula, the sponge manages to 

 bridge over the space between the two valves, and then the fate of 

 the mollusc is sealed. We have, however, observed an individual 

 of G. sandal in which the siphonal orifice was completely covered 

 by the growth of a sponge about three times as large as the shell, 

 but which yet remained not only alive but capable of movement. 

 Doubtless its inhalation aud exhalation of water took place through 

 the sponge's canal system. This condition of affairs could not 

 persist for long, as the sponge secretes a very thick horny 

 membrane at its base if irritated. 



The two other molluscs on which S. dementis, most commonly 

 grows are Vivipara sclateri and Melania multigranosa. In the case 

 of the former, in which the natural position of attachment is the 

 upper surface of the largest whorl ; but in the Melania, which, as a 

 rule holds its shell transversely, the position is near the middle of 

 the upper surface. In both cases the sponge spreads first over the 

 upper surface of the shell and then gradually round its sides, and 



