Fresh-water Sponges. 277 



proving that all the known existing species must have descended 

 from a much smaller number, if not from a single primitive 

 form. 



A few words now upon the function of the gemmules or stato- 

 blasts. 



Fresh- water sponges are much more liable than marine to 

 great variations of temperature. Ice would probably kill the 

 parent sponge. They are also liable to be left dry at certain 

 seasons, for Mr. Bates, in his book, ' The Naturalist on the 

 Amazons,' states that in one part of the river the difference in 

 level between the wet and dry seasons amounts to 25 or even 

 35 ft., that the floods last from three to four months, and when 

 the water retires " the trunks and lower branches of the trees 

 are coated with dry slime and disfigured by rounded masses of 

 fresh-water sponges, whose long horny spiculse and dingy colours 

 give them the appearance of hedgehogs." 



Another writer (Dr. Eusby) states that sponges on the River 

 Ibon are deposited on branches, and resemble the mud nests of 

 ants and bees. 



Some provision is therefore necessary for the continuance of 

 the species, and these gemmules are admirably adapted to with- 

 stand extremes of cold and drought, the outer crust of air-cells 

 being a good non-conductor. 



When placed under conditions favourable for germination the 

 protoplasmic contents " pass through the foraminal aperture, 

 spreading out on every side. In a few hours the infant colony 

 may be seen producing aqueous currents, developing and 

 arranging skeleton spicules, and in every way living the life of 

 a young sponge" (Potts). 



In preparing the spicules for microscopic examination, I have 

 found the most satisfactory way to be, to boil three or four 

 gemmules in a watch-glass with three or four successive drops 

 of nitric acid until they are perfectly disintegrated ; then treat 

 With liquid ammonia, which dissolves the altered animal matter, 

 and finally wash very carefully with distilled water. 



The gemmule spicules, together with gemmules made more 

 transparent by partial destruction with nitric acid, the normal 

 sponge and dermal film are all the elements necessary for deter- 

 mining the species. 



As might be expected, America has hitherto yielded by far the 

 largest number of known species, 58 or more being known, 

 besides varieties, and of these only about three or four have been 

 found in England. 



This subject is exhaustively treated in a monograph by Edwai'd 

 Potts, of Philadelphia, which has been of considerable use to me 

 in the preparation of this commimication. 



