The Sponges of Lake Biwa. i g 



the sponge is characteristic. No two species growing side by side 

 in the same environment are liable to be confused, but often the 

 same species may assume a very different appearance in different 

 environments, sometimes even in an environment that seems, 

 at any rate on a superficial examination, to be homogeneous. In 

 this respect S. dementis is the most interesting sponge found in L. 

 Biwa, but it may be conveniently considered from the point of 

 view after others in which the environmental variation is less 

 strongly marked. 



S. semispongilla, as has already been pointed out, occurs in 

 two very distinct phases, one of which is a mere film of very 

 limited area, while the other forms a compact, shrub-like growth. 

 The dwarf phase has been found only on the flat transverse 

 leaves of water plants, but the bushy phase grows either on stones 

 or on water-weeds with small leaves. A very profuse growth 

 occurred in October and November, 1915, on stones near the edge of 

 the lake close to the Otsu station. The sponge had much the 

 appearance of the compact tufts produced by many rock-loving 

 plants. S. lacustris produces when encrusting reed-stems much 

 more delicate branches than it usually does when growing on 

 stones and posts. The growth of S. frag ills on stones is always 

 feebler than on cylindrical bodies. In the former position it is often 

 found on the lower or protected surface and is thus the only sponge 

 in the lake-fauna that shows any tendency to conceal itself either as 

 a protection against sunlight, against mud, or against enemies. In 

 some lakes {e.g. the Lake of Tiberias in Palestine) this tendency is 

 well marked in the case of most sponges, so that a superficial ex- 

 amination would seem to indicate an almost complete absence of 

 sponge-fauna, but in L. Biwa such species as S. semispongilla, S. 

 lacustris, S. dementis and less commonly E. m'dlleri var. japonica 

 are very conspicuous objects in calm weather. The absence of sus- 

 pended silt in the water is probably responsible to a very consider- 

 able extent for this striking feature. 



It is probable that the three phases of S. dementis are correlated, 

 or may be correlated, in all cases with environment, but Phase I 

 and Phase II are sometimes found growing together, and indeed 



