16 _ SPONGES 
COLOR OF FRESH WATER SPONGES. 
As intimated, most of the Iresh Water Sponges are dull in cotor, the. 
brightest among them being a rather bright green. ‘Phis greenish color appears. 
to vary somewhat with individuals. Those which grow im situations where, 
they are exposed to the full rays of the sun are the brightest, while those. 
even of the same species, which graw in shady places are sometimes nearly 
white. ‘The large, flat species of Spongilla aye brown in color, and have been, 
spoken of by writers as yesembfing masses of liver, 
LIMY SPONGES, 
In these sponges the spicules are eomposed wholly of carbonate of lime, 
dut do not differ greatly in other respects from Salt Water Spiculigenous 
Sponges, ‘They are usually small in size, often noé aver an inch in length, 
and are rather simple jn strueture, See Fig. 40, where I have given an ex- 
ample of a Limy Sponge of the genus Grantia, A, being the sponge entire, 
and B ascction where the spicules may be seen projecting from the ends. 
As will be seen, the water system js very simple, the incurrent tubes open- 
ing into a civcufar system of tubes which pass directly through to the ctoacak 
or central tube. See Fig. 41, where I give a section of one of these sponges, 
areatly enlarged. A is the incurrent tube, B, the excurrent tube opening into 
the large central tube, 
