v4 SPONGES 
GEOGRAPHIGAL RANGH OF SILICIOUS SPONGES. 
The geographical range of sponges of this group is much more extended 
than that of the Horny Sponges, for they are found frem the Arctic to the Ant- 
arctic Circle, but are far more abundant in Tropical and sub-tropical waters 
than elsewhere. Inthe West Indjes, for example, the species outnumber 
those of the Horny Sponges, 
FRESH WATER SILICIOUS SPONGES. 
If we scarch in fresh water streams, ponds, or ditches, which do not become 
(ry in summer and which are plentifully supplied with pure spring water 
which is net hard, that is too much impregnated with carbonate of lime, we 
shall usually find clinging to the sides of submerged stones, to sticks, stumps, 
ete. greenish or brownish masses which have a strong, but not to many, a dis- 
agreeable odor, These are Fresh Water Sponges. Their forms are variable, 
from delicate branching structures, only an inch or two in length, to large, 
flattened masses, often a foot or more in djameter, and two or three inches in 
thickness. 
Under a microscope with moderate power, say fifty diameters, these 
sponges will be found to be made up of bundles of spicules, which are usually 
spindle shaped. See Fig. 82, P, page 46, also Fig. 39, C, where I have giv- 
en the spicules taken from a member of the genus Spongilla, which was collec- 
ted in a pond in Newtonville. In the figure last mentioned, the spicules are 
