The Sponges of Lake Biwa. 21 



greater part of which has a muddy bottom, but is found only in 

 one part of the lake, where the mud is washed away by the cur- 

 rents of the R. Gordam passing through the lake, and sweeping- 

 clean a bed of fine gravel not unlike that on which Phase III of S. 

 dementis is commonly found in L. Biwa. In both the species 

 minute stones are found imbedded in the sponge and covered with 

 a horny laydr, evidently secreted to protect the soft parts from 

 contact with them. This proves not only that the sponge engulfs 

 and has difficulty in expelling such particles, but also that their 

 presence is undesirable. 



Although Phase III of S. dementis is abundant on the Corbi- 

 eula-beds of the North Lake, we did not find it in its full 

 development on them in the South Lake. On the beds in the 

 latter, small thin encrusting specimens were, however, occasionally 

 seen on Corbicula-shells. The fact is noteworthy, because some of 

 the richest of the beds in L. Biwa lie close to Seta bridge, on which 

 the less massive phases of the same sponge grow luxuriantly. The 

 absence of the massive phase in this region may be correlated with 

 the greater impurity of the water in the neibourhood of very luxu- 

 riant aquatic vegetation. It is not wholly due to the more muddy 

 character of the bottom, for the sponge is very common off Hikone 

 and in the channel between the Island of Okinoshima and the 

 main land, in both of which places the bottom is of a somewhat 

 mixed character composed both of larger and finer particles. 



The colour of the third phase of S. dementis varies somewhat 

 on different kinds of bottom. When the bottom consists of coarse 

 sand or fine gravel it is almost white, while a mixture of sand or 

 gravel with fine mud produces a greyish tinge. If grey sponges are 

 kept, however, for a few days in a vessel of clean water they be- 

 come quite white, thus showing that the grey tint is due to the 

 absorption of microscopic particles of mineral matter. 



Seasonal variation is a marked feature of several of the sponges 

 of L. Biwa. Only its grosser manifestations have as yet been in- 

 vestigated, but the cytological changes promise a rich field fur 

 inquiry, which we hope will not be neglected in the future. 1 ' 



1) cf. Weltner : Arch. Xaturg. Berlin, LXIII, P. 273 (1907). 



