58 SPONGES 
land. The thickness varies from .10 to .50 of an ineh. See plate V, where 
I have given the figure of a fine specimen of the Green Incrusting Sponge, Ter- 
pios fugax D. and M. . 
ODOR OF SPICULIGENOUS SPONGES. 
Although these sponges have an odor which somewhat resembles that of 
the Horny Sponges, it is seldom as strong. The Palm and Common Branching 
Sponges have a rather sweetish scent which is not unpleasant, while the Purple 
Sponge has an odor which is not disagreeable. 
HABITS OF SALT WATER SPONGES. 
These sponges oceur in all depths of water, from between tide marks to 
fifty feet or more deep. Some species are habitual parasitical, others like the 
Purple Sponge, occasionally grow on other sponges, but as far as my experience 
goes, Horny Sponges are always.selected asa base of growth, for I have never 
seen a sponge of any species growing upon a Spiculigenous Sponge. 
Another peculiarity observed in members of this group, 1s that they do not 
appear to display much, if any, perceptible sensitiveness upon being handled : 
in fact, I have never seen any species of Silicious Sponge close the excurrent 
orifice, even when removed from the water. 
A matter which is of great interest inregard to this group of Spiculige- 
nous Sponges, is the power which they possess in the spicules asa protective 
medium. As shown, the spicules seem to afford them protection against the 
