DESCRIPTION OF A FRESHWATER SPONGE FROM THE NORTH-WEST OF SIBERIA. 
3 
Svartzewsky’s figure 1 ), to resemble Ephydatia olchonensis from Lake Baikal. Evans 2 ) 
lias sliown that Spongilla moorei from Lake Tanganyika possesses a skeleton that is similar 
in some respects, especially as regards the manner in wliich the cliitinoid investment of the 
upright spiculé fibres is in continuity with the basal membrane. The spiculés are very like 
those of Spongilla lacustris , except for their small size and for the great range of variation 
exhibited by the megascleres as regards both size and relative thickness. 
Explanation of figures. 
1. Vertical section of a portion of the skeleton of S. arctica from near the external 
surface of the sponge, showing the vertical spiculé fibres and free megascleres, x 70. 
2 . Vertical section of a piece of the skeleton from the middle of the sponge, more 
highly magnified. A = gemmule spiculé; B = chitinoid investment of the spiculés. 
3. The base of a vertical spiculé fibre, showing its connection with the basal mem¬ 
brane, x70. 
4. Part of the cage of megascleres investing a gemmule, with part of a spiculé fibre 
and a single free megasclere, x70. 
Til[ LT f H[ 
JUN i ; ipjj 
1) Zapiski Kiev. Obsch. XVII (2), p. 14, pi. IV, fig. 12 (1901). Sec, however. Korotneff (Biol. Centrbl. Berlin 
XXI, p. 306, 1901), who says, «Spongin ist sehr spärlich vertreten». 
2) Q. J. Micr. Sei. XII, p. 474 (1899). 
