70 SPONGES 
have never found elsewhere than in sponges. ‘This shell begins life as a spiral 
gasteropod, but after growing to the length of a half inch or a little less, begins 
to send upa tube much like that occupied by some species of marine worms, 
the entrance of which the animal appears to be obliged to keep upright, hence 
it avails itself of the support afforded it by some species of sponge ; as the fibers 
and flesh of the Scarlet Sponge are soft, the worm shells select it as a dwell- 
ing place. The Mollusk adds to its tube as the sponge grows, thus its entrance 
is always kept on a level, or a little in advance of the upper surface of the 
sponge. The sponge clings so closely to this seeming intruder that the shell 
cannot be removed without tearing away some of the sarcode and fiber of the 
sponge. Thus the sponge finds the support, which it desires in the shells, 
nullipores ete. which forma kind of skeleton for it, while the Mollusk finds the 
sponge admirably fitted to keep its crooked shell upright ; thus sponge and shell 
are of mutual benefit, and I have found as many as six worm shells in a 
single small Scarlet Sponge. 
The Branching Sponge of our coast grows attached to bridges or wharf 
piles, usually just below low water mark or deeper, or to stones at greater 
depths. They are usually found in upright branching, anastomosing clusters. 
The two Cup Sponges, figured on page 38, occur in from twenty to thirty- 
five feet of water, almost always in tide ways, but usually where there is some 
obstruction like a small coral reef or bank, which breaks the full force of the 
tide. The Green Cup has a slight attachment for the rocky base upon which 
it grows, but the Gray Cup is more firmly attached. 
