SPONGES 75 
seen to be hollow, which is the natural condition of them in most of the sponges. 
If these sponges be gathered in late autumn or winter, they will be found 
to be provided with seed-like bodies which lie within the sponge, or at its base 
and which can easily be seen by the aid of a common magnifying glass. 
These are the winter oyules, and they are surrounded by peculiar spicules, 
sometimes bent, one of which highly magnified, I have given on page 46, Fig. 
31, A, and at Bis given some of the ovules considerably enlarged, Another 
gemule spicule is given jn Fig. 39, D, 
Upon a microscopic examination of these ovules, they will be found to be 
made up of minute ova which jn the spring develop into ciliated animals, which, 
bursting the ovule case, swim about freely for a time, but eventually become 
fixed and produce other sponges. 
The water system of Fresh Water Sponges is essentially the same as 
that given already in the Salt Water Silicious Sponges, but it is eyen more 
difficult to make out, and there js little or no horny fiber, | 
HABITS OF FRESH WATER SPONGES. 
I have found the Green and Branching forms of Fresh Water Sponges 
erowing on sticks in the bottom of ditches, or clinging to the sides of stones i1 
ponds. | Upon examining the bed ofa mill pond in Newtonville after the 
water had been drained off, I found large, flattened masses of a species of Spon- 
gilla a foot or more in diameter, clinging to the roots of some willows, which, 
as is often customary with these trees, had sent their roots out into the water 
in large clusters, 
