234 FRESH WATER SPONGES OF NOVA SCOTIA— MACKAY. 
definitely distinguished between the two known forms at that 
time, naming them respectively Spongilla lacustris and fluvia- 
tilis (now, Meynia fluviatilis). The macroscopic characters of 
fresh water sponges are often so variable or ill-defined that a 
microscopic examination of the general structure of the repro- 
ductive gemmule which Carter denominates a “statoblast,” is 
necessary to make sure of its identity. Accordingly, Carter, in 
1881, divided the fresh water sponges then known to science 
into five genera, each of which is distinctly characterized by the: 
spiculation. 
In Canada, Sir William Dawson, of McGill, in 1863, sent a 
specimen of freshwater sponge to Bowerbank, who described it 
under the name of S. Dawsoni. In 1875, George M. Dawson, 
son of Sir William, described four additional sponges as new to 
Canada and science. Although these are not likely to rank as: 
species in the future system of classification, they mark a good 
step in the advance of our knowledge of these organisms by one: 
whose energy and encyclopedic knowledge of Canadian Natural 
History have done much, and promise to do still more, in bring- 
ing our Dominion under the notice of the scientific world. 
Our freshwater sponges are, so far as observed, generally 
greenish, poriferous or variously channelled masses, of a rather 
soft,but not fluid sarcode, supported by a skeleton of silicious spi- 
cules, or needles, approximating the one hundredth of an inch in 
length, variously combined to form a mesh-work structure, which. 
may assume the contour of thin or thick encrusting layers, even 
or lobed, in some species branching erect, or creeping in slender 
filaments. The statoblasts which appear to mature before win- 
ter, commonly in the basal portion of the sponge, are more or less 
spherical, stsaller than the head of a pin, variable as to size and 
mode of aggregation, and contain a mass of free cellular germinal 
matter, which is enclosed by a strong chitinous membrane, with 
a small variously formed and directed aperture. The chitinous. 
coat is surrounded by an outer structure, generally densely 
charged with a regularly arranged investing layer of character- 
istic spicules, in most species bearing no resemblance either im 
form or size to the skeletal spicules. 
